ALLOCATION OF TOPICS User Guides2c053ed6bf52929a.jimcontent.com/.../name/SCE3105.pdf · SCE3105...

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ALLOCATION OF TOPICS User Guide

The content of this module will replace one credit which is equivalent to fifteen hours face-to-face interaction. The table below will clarify the allocation of topics for face-to-face interaction or learning by module. (Allocation of topics by face-to-face interaction and module based on the course pro forma)

Bil.

Title/Topic

Face-to-Face

Interaction (hour)

Module (hour)

Total no. of

Hour

1 Physics and Measurement in Everyday Life

4* 2 6

2 Motion – in which direction?

- 2 2

3 Motion in one dimension 4* 2 6

4 Motion in two dimensions 1 1 2

5 Applying Newton’s laws in everyday life - 2 2

6 Work and machines

2+2** - 4

7 Forces in fluids 2** 2 4

8 Planetary and satellite motion

2 - 2

9 The physics of music

2+4* - 6

10 Thermometry and thermometers

2** 2 4

11 Using Light 4* 2 6

12 Electrical circuits in the house

2+4* - 6

13 Electricity and magnetism at work

2 - 2

14 Generation and transmission of electricity

2+4* - 6

15 Use of electronics and semi-conductors

2 - 2

Total 45 15 60

* Amali **PCK

SCE3105 Physics in Context

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TOPIC 1

PHYSICS AND MEASUREMENTS IN EVERY DAY LIFE

Synopsis The physical science is based on principles and development of concepts. The application of principles and concepts usually involve one or more physical quantities. Almost the whole world is using the metric system in everyday life. One adaptation of the metric system is used in science, business and communication. This system is known as the SI system (System International). In this topic, you will be exposed to the S.I. units, its conversions, scientific notation, accuracy and precision of measurements, significant digits as well as techniques of good measurements. Learning Outcomes

1. Convert measurements from one unit to another.

2. Write very large or very small physical quantities in scientific notation

3. Write physical quantities to the proper significant figure.

4. State the techniques of good measurements.

Overview

Figure 1.1 Overview of content

Physics and measurements

in everyday life

Conversion of units

Scientific notation

Accuracy and precision

Significant digits

Techniques of good

measurements


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CONTENT 1.1 Conversion of units Similar to the number system, the metric system is based on the decimal system. Prefixes are used to convert the SI unit into the power of ten. For example, one tenth of a meter is a decimeter and one hundredth of a meter is a centimetre. The metric unit uses the same prefixes for all quantities. For example, one over one thousand gram is a milligram, and one thousand gram is a kilogram. To use the SI unit effectively, it is important that you know the meaning of each prefixes as shown in Table 1.1.

Table 1.1

Tutorial 1 (½ hour)

Surf the following web pages to gather information about the SI units and the historical development of the SI units. Summarize your understanding in your reflective notebook. http://www.bipm.org/en/si/ http://en.wikipedia.org/wiki/SI

Prefixes Value Standard Unit Symbol

Tera 1 000 000 000 000 1012 T

Giga 1 000 000 000 109 G

Mega 1 000 000 106 M

Kilo 1 000 103 k

Desi 0.1 10-1 d

Centi 0.01 10-2 c

Mili 0.001 10-3 m

Mikro 0.000 001 10-6 µ

Nano 0.000 000 001 10-9 n

Piko 0.000 000 000 001 10-12 p


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Example:

What is the equivalent value of 500 milimeters in meter?

Answer:

From Table 1.1, the conversion factor is

1 milimeter = 1 x 10-3 meter

Therefore, 500mm is equivalent to

(500 mm) mm

m1101 3

= 500 x 10-3 m = 5 x 10-1 m.

Exercises

1. Convert the length below to its equivalent value in meter a. 1.1 cm b. 56.2 pm c. 2.1 km d. 0.123 Mm 2. Convert the mass below to its equivalent value in kilogram a. 147 g b. 11 µg c. 7.23 Mg d. 478 mg 1.2 Scientific Notations

The investigation in science sometimes involve quantities which are very small or very big. For example, the mass of earth is about

6 000 000 000 000 000 000 000 000 kilogram and the mass of electron is 0.000 000 000 000 000 000 000 000 000 000 911 kilogram The quantities written in this form take lots of space and are difficult to use in calculations. To make it simpler to calculate using these values, we write them in a shorter form by substituting the decimal places with numbers with the base ten. Scientific notation is M x 10n

where 1≤ M ≤ 10 and n is an integer


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Therefore the mass of earth can be written as 6.0 x 1024 kg and the mass of electron as 9.11 x 1031 kg. The magnitude of certain quantities are usually converted to the nearest three of four significant places. 1.3 Significant digits Due to the limited sensitivity of a measuring instrument, the valid number of digits are limited. These valid digits are called significant digits. The number of significant digits in a measurement can be determined by referring to the following statements: 1. Non zero digits are always significant. 2. All final zeros after a decimal point are significant. 3. Zeros between two other significant digits are always significant. 4. Zeros used to give space to decimal point is not significant.

Thinking

1.4 Accuracy and precision Precision is the degree of exactness of a measurement. For example, if a student has conducted an experiment to measure the speed of light, he will repeat his measurements a few times. A few attempts produced values between 3.000 x 108 m/s to 3.002 x 108 m/s where the average value is 3.001 x 108 m/s. He concluded that the speed of light is 3.001 x 108 m/s. From these measurements the speed of llight has a range of 3.000 x 108 m/s to 3.002 x 108 m/s. Therefore the accuracy of the measuring instrument is 0.001 x 108 m/s. Precision of a measuring instrument is limited to the smallest reading of the measuring instrument. Accuracy describes how well the results of an experiment agree with the standard value. In an experiment to measure the speed of light, accuracy is the differences between the measured values stated with the same precision. For example, student’s measurement is 2.998 x 108 m/s as compared to the standard value of 3.002 x 108 m/s. Therefore the accuracy is 0.003 x 108 m/s.

How do you add, subtract and multiply numbers with significant digits? Refer to mathematical methods to do the operations. .


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Figure 1.2 Instruments for the measurements of length

1.5 Technique of good measurements In a physics experiment, measurements should be precise and accurate. The following steps should be considered for good measurements 1. Choose a relevant measuring instrument for one measurement (a) An error of 0.1 cm in a measurement 100.0 cm is not a too serious error compared to an error of 0.1 cm in 10.0 cm.

(b) Measurements with large values like length of a wire do not need a sensitive

instrument, whereas measurement of a small values like the diameter of a wire will require a sensitive instrument.

2. Accurate measurement using the measuring instrument (a) Always follow the instructions given for operating the measuring instrument. (b) Be careful while making measurements. (c) Understand the different form of errors.

Discussion

Discuss the appropriate measuring instruments to measure the following physical quantities: length of rope, thickness of a piece of paper, thickness of a window pane, thickness of a book and width of a table.

References http://www.bipm.org/en/si/ http://en.wikipedia.org/wiki/SI (System Internationale)


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TOPIC 2

MOTION- IN WHICH DIRECTION?

Synopsis The movement of objects could be represented by words used in our daily life such as distance and speed. In physics, we use new words like displacement, velocity and acceleration to represent movement. The difference between these two categories of words is in term of their quantities, which are the vector and scalar quantity. In this topic, you will be exposed to the addition and subtraction of vectors, relative velocity and resolution of vectors. Learning Outcomes

1. Describe how to represent vector quantities. 2. Add or subtract vectors using the graphical method. 3. Determine the relative velocity using addition and subtraction of vectors. 4. Resolving vectors into its components. 5. Add vector algebriacally.

Overview


Movement

Vector Scalar

Addition and subtraction of

Graphically

Algebraically

Components of vectors

Relative velocity


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Content 2.1 Representing vector quantities A vector quantity is represented by a line with an arrow at its end. The length of the line is drawn according to scale to represent the magnitude of the quantity. The direction of the arrow shows the direction of the quantity. Other than representing a vector graphically, we can also add two vectors graphically. Vectors are represented by the letters in the alphabet such as A or B.

2.2 Addition of vectors Addition of vectors in one dimension

Figure 2.2

If a girl is moving 200 m east, and then another 400 m east, her total displacement is the sum of the two vectors. Vectors A and B are drawn as scaled as shown in Figure 2.2(a). Therefore the magnitude resultant vector, R = A + B or, R = 200m + 400m =600 m, and the direction of resultant vector A and B is 600m east. Look at Figure 2.2(b) dan 2.2(c). Think how you can find the resultant vector graphically for these two cases.


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Addition of vectors in two dimensions To add two vectors, refer to Figure 2.3 and draw scaled diagrams.

Scale: 1 cm represent 20 N

Figure 2.3

What is the magnitude of the resultant vector, R ?

Reading Materials

Refer to this webpage to read further on addition of vectors http://phet.colorado.edu/sims/vector-addition/vector-addition_en.html

2.3 Subtraction of vectors in one dimension To subtract two vectors, you have to find the addition of two vectors which are in opposite directions (Figure 2.2c). The magnitude of resultant vector, R = A + (-B) 2.4 Relative velocity : a few applications Sometimes, object moves in a medium which is moving relative to the observer. An aeroplane moving in one direction will experience a change in velocity and direction due to the movement of air (wind).

Step 1: draw parallelogram

Step 2: draw resultant vector

A

B

R


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Velocity of aeroplane + wind velocity = Resultant velocity Relative velocity = 100 km/hr + 25 km/hr = 125 km/hr

Find the relative velocities of the aeroplane in the situations given below. (a) Wind velocity 25 km/hr north (b) Wind velocity 25 km/hr west

Reading Materials

2.5 Components of vectors The components of vectors are the parts of a vector, in most situations there are the x-component and the y-component.

Figure 2.4

The red arrow at Figure 2.4 shows the x-component of vector F and the blue arrow shows the y-component of vector F. Using trigonometri, the x-component, Fx is F cos while the y-component, Fy is F sin ө.

35◦

F = 316N Fy

Fx

Refer to this webpage to see the applications of vectors to find relative velocity http://physics.bu.edu/~duffy/java/RelV2.html


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2.6 Algebraic addition of vectors The addition of two could be done by using trigonometry by using the sides of a right angle triangle. Two mathematical formulae used are: (1) Trigonometry

(2) Pythagoras Theorem

Example: Find the addition of the two vectors below

Answer: Step 1 : To complete a right angle triangle

Step 2: Use the Pythagoras theorem to find the magnitude of resultant vector.


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Step 3: Use trigonometri to find the angle that represents the direction of resultant vector

Tutorial 2 (1 hour) Refer to Tutorial_Topic 2 pdf file to practice on calculations for this topic. Modul SCE3105 Phy In Context 22Oct'09\Tutorial_Topic 2.pdf

References: http://physicslearningsite.com/vectors.html http://phet.colorado.edu/sims/vector-addition/vector-addition_en.html (addition and subtraction of vectors) http://id.mind.net/~zona/mstm/physics/mechanics/vectors/findingComponents/findingComponents.htm (vector components) http://www.glenbrook.k12.il.us/gbssci/Phys/Class/vectors/u3l1f.html http://physics.bu.edu/~duffy/java/RelV2.html (relative velocity)

TOPIC 3

MOTION IN ONE DIMENSION

Synopsis Movement of an object will result in a change their position. The change in position of an object is due to the application of a force. Movement is usually described in terms of velocity, acceleration, displacement and time. The velocity of an object could not change unless it is acted by a force. In this topic, you will be exposed to the ways of describing motion, equations of motion and types of forces. Learning Outcomes

1. State the a few ways to describe motion which is in one dimension 2. Discuss the types of forces: friction, normal, tension, upthrust and weight. 3. Discuss the action of the forces in different context.


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Overview

Figure 3.1 Overview of content Content Motion can be described by:

Description Example Words

Distance: Distance between two points Displacement: How far away from the original point Speed: A scalar quantity to describe ‘how fast is the object moving’ Velocity : A vector quantity to describe the ‘rate at which an object change its position’ Acceleration: A vector quantity to state ‘the rate an object change its velocity’

Graph

Motion in one dimension

Kinematic Dynamics Forces

Description of motions Contact forces Long Range force

Friction Gravitational force

Normal

Tension

Upthrust

Words

Figures

Graph

Formula


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(i) Ticker tape diagram

Figures

(ii) Vector diagram

Kinematic formula v = u + at v2 = u2 + 2as s = ut + ½at2

Kinematic graphs

Reading Materials

s displacement u initial velocity v final velocity a acceleration

Refer to the following webpage to see a few ways motion can be described: http://www.physicsclassroom.com/Class/1DKin/


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Ffriction µ x Fnormal where µ = coefficient of friction geseran

3.1 Types of force Force is the pull or push of an object which causes it to interact with another object. When this interaction decrease, the object will no longer experience a force. Force only exist due to interaction. Force is measured in the SI unit Newton. One Newton is equivalent to 1 kgms-2. Force is a vector quantity. It has both magnitude and direction. A few forces between object that will be discussed are:

Types of forces Frictional force

Gravitational force (weight)

Normal force Tension Upthrust

Frictional force Frictional force is the force acting on the a surface when trying to move across it. It usually act at a direction opposite to the direction of movement. There are two types of frictional force which are the static friction and sliding friction. Friction is caused by two surfaces pushed together, causing an attractive force between molecules from different surfaces. It is dependent on the types of surfaces and how hard is the object is pushed. The maximum friction can be calculated by the following formula: Gravitational force (weight) Gravity is the force that pulls objects downward toward the the earth. Objects falling to the earth without the influence of external forces (like air resistant) is said to experience free fall. These objects will have an acceleration called the gravitational acceleration. Weight is the attractive force of the earth towards the object. If the object has a mass, m and gravitational acceleration g, its Weight = mg. Normal force Normal force is the supportive force on an object when it touches a surface. For example, if a book is placed on a table, the surface exert a force upwards to support the weight of the book (Figure 3.2a). It can also exist horizontally when two objects are in contact with one another. For example, a person leaning on a wall (Figure 3.2b) will


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exert an horizontal force to the wall. Thus, the wall exert a normal horizontal force in the opposite direction (Rajah 3.2b).

(a) (b)

Figure 3.2 Tension Tension is the force which is transfered through ropes, strings, cables or wires when a force is applied at both ends of the ropes, strings, cables or wires. Tension acts at both ends of the rope and they are equal and opposite to the force applied at the ends of the rope. Upthrust Upthrust is the force which pushes the object upwards and causing it ‘lose weight’ in fluids (liquid or gases). It can also cause an aeroplane to move through the air.

Reading Materials

References

http://www.physicsclassroom.com/Class/1DKin/ (Description of motion) http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/newtlaws/u2l2a.html http://www.zephyrus.co.uk/forcetypes.html (Types of forces and its application)

Refer to the webpage below to find out more about these forces and their applications: http://www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/newtlaws/u2l2a.html http://www.zephyrus.co.uk/forcetypes.html


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TOPIC 4

MOTION IN TWO DIMENSIONS

Synopsis Motion is an action which can be observed everywhere in daily life. When a force act on something, it will cause it to move or stop it from moving. Force is a vector, the motion produced by a force will be in the same direction as the force. To produce a motion in two dimension, there must be at least two forces acting in two different direction. In this topic, you will be exposed to the concept of equilibrium of forces in two dimensions and motion of object on an inclined plane. Learning Outcomes

1. State the conditions for an object acted on by three forces to remain in equilibrium. 2. Determine the force that produces equilibrium when three forces act on an object 3. Analyse the motion of an object on an inclined plane

Overview


Content 4.1 Equilibrium and the Equilibrant An object is in equilibrium when the net force acting on it is zero. When in equilibrium, an object is motionless or moves with constant velocity. Equilibrium also occurs when the resultant force of three or more forces equals zero (net force equals zero).

Figure 4.2(a) shows three forces A, B and C exerted on a point object. What is the sum of the three forces or what is the net force acting on the point object?

Motion in Two Dimensions

Condition for Equilibrium Equlibrium Resultant Force

Motion on an Inclined Plane


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Figure 4.2(b) shows the addition of the three forces A, B and C. Note that the three vectors form a closed triangle. There is no net force so the sum is zero and the object is in equilibrium.

(a) (b)

Figure 4.2 An object is in equilibrium when all the forces on it add up to zero . (Source: Physics: Principles and Problem)

Suppose two forces L dan M,(Figure 4.3(a)) are exerted on an object and the sum is not zero. How can you find a third force, when added to the other two, would add up to zero? Such a force, one that produces equilibrium, is called the equilibrant. To find the equilibrant, first you must find the sum of the two forces exerted on the object. This sum is the resultant force, R (Figure 4.3(b)) the single force that would produce the same effect as the two individual forces, L dan M added together. The equilibrant is thus a force with a magnitude equal to the resultant force but in the opposite direction(Figure 4.3(c)).

(a) (b)

(c)

Figure 4.3 The equilibrant is a force with the same magnitude as the resultant force but in opposite direction.


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Exercise

Surf the internet and try to answer the questions on the equilibrium of forces. The website below is one of the sources where you can carry out interactive exercises on equilibrium of forces.

http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter5/interactive_tutor.html

4.2 Motion Along an Inclined Plane

All objects on Earth will experience a gravitational force directed toward the centre of the Earth. For an object located at the slope of a hill, what forces, besides the gravitational force, act on the object? Figure 4.4 shows the forces acting on an object placed on a slope of a hill, or on an inclined plane.

Figure 4.4 Forces acting on an object on an inclined plane.

In Figure 4.4, N is the normal force perpendicular to the inclined plane. F is the frictional force which acts parallel or along the inclined plane. The direction of frictional force is opposite to the direction of motion which is down the inclined plane. When the object is at rest at the inclined plane, the forces acting on the object are as shown in Figure 4.5.

Figure 4.5


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To facilitate the study of motion of object along an inclined plane, a suitable coordinate system needs to be established. Since the direction of motion of the object is parallel to the inclined plane, one axis (usually the x-axis) is be along the inclined plane. The y-axis, as usual is perpendicular to the x-axis or to the inclined plane. For such a coordinate system, the normal and frictional forces are both in the direction of the coordinate axis, but the weight is not. In most problem, you will have to find the x and y components of this force as shown in Figure 4.6.

Figure 4.6

Thinking

Access the website below to learn from the animation of a skier with detail of the forces acting on it: http://glencoe.com/sec/science/physics/ppp_09/animation/Chapter%205/Motion%20Along%20an%20Inclined%20Plane.swf Based on the information gathered from the website above, construct a free-body diagram to show the forces acting on an object moving down an inclined plane.


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The free-body diagram shows the forces acting upon a 100-kg crate which is sliding down an inclined plane. The plane is inclined at an angle of 300 . The coefficient of friction between the crate and the incline is 0.3.

Determine the net force and acceleration of the crate.

References http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter5/ (forces in two dimension) http://glencoe.mcgraw-

hill.com/sites/0078807220/student_view0/chapter5/interactive_tutor.html (equilibrium of forces) http://glencoe.com/sec/science/physics/ppp_09/animation/Chapter%205/Motion%20Along%20an%20Inclined%20Plane.swf (Forces on an inclined plane) Zitzewitz,P.W.(2002) Physics: Principles and Problems. Ohio: Glencoe/McGraw-Hill.

Tutorial 3 (½ hour)

SCE3105 Physics In Context

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TOPIC 5

APPLYING NEWTON’S LAW IN EVERYDAY LIFE

Synopsis Forces are so common and they can be seen or felt often in our daily life. A force can make something move and it can also make something in motion to come to a stop. There are many types of forces which can be classified into two main which are the contact force and long-range force. Forces play a vital role in the study of motion. Isaac Newton build on from Galileo’s work to formulate three simple laws governing motion. In this topic, you will be exposed to Newton’s three laws of motion and how these laws help us to understand the phenomena of force and motion in our everyday life. Learning Outcomes

1. Define force and differentiate between contact forces and long-range forces. 2. Explain the meaning of Newton’s first law. 3. Recognise the significance of Newton’s second law. 4. State Newton’s third law and use it to explain forces come in pairs. 5. Apply Newton’s laws to solve problem on forces and motion. Overview


Content 5.1 Contact and Long Range Forces A force is a push or pull upon an object resulting from the object's interaction with another object. Whenever there is an interaction between two objects, there is a force upon each of the objects. When the interaction ceases, the two objects no longer experience the force. Forces only exist as a result of an interaction. For simplicity sake, all forces (interactions) between objects can be placed into two broad categories:

contact forces, and

forces resulting from action-at-a-distance(Long-range forces)

APPLICATION OF NEWTON’S LAWS

Contact & Long Range Forces

Newton’s laws Application of Newton’s laws


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Contact forces are those types of forces which result when the two interacting objects are perceived to be physically contacting each other. Examples of contact forces include frictional forces, tensional forces, normal forces, air resistance forces, and applied forces. Action-at-a-distance forces or long-range forces are forces which result when the two interacting objects are not in physical contact with each other, but yet able to exert a push or pull despite their physical separation. An example of action-at-a-distance forces is the gravitational force. The sun and planets exert a gravitational pull on each other despite their large spatial separation. Even when your feet leave the earth and you are no longer in physical contact with the earth, there is still a gravitational pull between you and the Earth. Electric force is another example of action-at-a-distance force. The protons in the nucleus of an atom and the electrons outside the nucleus experience an electrical pull towards each other despite their small spatial separation. Another example is the magnetic force. Two magnets can still exert a magnetic pull on each other even when they are separated by a distance of a few centimeters. Examples of contact and action-at-distance forces are listed in Table 5.1.

Contact Forces Long Range Forces

Frictional Force Gravitational Force

Tension Force Electrical Force

Normal Force Magnetic Force

Air Resistance Force

Applied Force

Spring Force

Table 5.1

Force is a quantity which is measured using the standard metric unit Newton. A Newton is abbreviated by a "N." To say "10.0 N" means 10.0 Newtons of force. One Newton is the amount of force required to give a 1 kg mass an acceleration of 1 ms-2. In addition to types of forces mentioned in Topic 3, there are other types of forces as listed in Table 5.2.


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Type of Force (Symbol)

Description of Force

Applied Force (Fapp)

An applied force is a force applied to an object by a person or another object. If a person is pushing a desk across the room, there is applied force acting on the object. The applied force is the force exerted on the desk by the person.

Air Resistance (Fair)

The air resistance is a special type of frictional force which acts on objects as they travel through air. The air resistance is often opposed the motion of the object. This force is frequently be neglected due to its negligible magnitude (and the fact that it is mathematically difficult to predict its value). It is most noticeable for objects which travel at high speeds (e.g. a skydiver or a child going down a slide in the playground) or for objects with large surface areas.

Spring Force (Fspring)

The spring force is the force exerted by a compressed or stretched spring on any object which is attached to it. An object which compresses or stretches a spring is always acted by a force that restores it back to its rest or equilibrium position. For most springs (especially those that obey "Hooke's Law"), the magnitude of the force is directly proportional to the amount of stretch or compression of the spring.

Table 5.2

Reading Materials

Please press ctrl+click on the pdf file below

Modul SCE3105 Phy In Context 22Oct'09\Explaining Motion.pdf (or refer Explaining Motion PDF file) Read the notes on forces in Page 2, 3 and 4.

5.2 Newton’s Laws of Motion Newton First Law of Motion Newton's first law of motion stated that

An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

There are two parts to this statement - one part predicts the behavior of stationary objects and the another predicts the behavior of moving objects. The two parts are summarized in Figure 5.2.

Figure 5.2


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The behavior objects is such that objects tend to "keep on doing what they're doing" unless acted upon by an unbalanced force. An object which is initially at rest will continue to be in state of rest. If it is in motion with an eastward velocity of 5 m/s, it will continue in this same state of motion (5 m/s, east). The state of motion of an object is maintained as long as the object is not acted by an unbalanced force. All objects resist changes in their state of motion - they tend to "keep on doing what they're doing." Suppose that you filled a baking dish to the rim with water and carried it around in an oval track, trying to complete a lap in the least amount of time. The water would have a tendency to spill at specific locations on the track because:

the container was at rest and you attempted to move it the container was in motion and you attempted to stop it at specific locations. the container was moving in one direction and you attempted to change its

direction. Everyday Applications of Newton's First Law There are many applications of Newton's first law of motion. Have you ever observed the behavior of coffee in a coffee cup filled to the rim while starting a car from rest, or while bringing a car to rest from a state of motion? The coffee tends to "keep on doing what it is doing." When you accelerate a car from rest, the road provides an unbalanced force on the spinning wheels to push the car forward; yet the coffee (initially at rest) wants to stay at rest. While the car accelerates forward, the coffee try to remain in the same position. When the car accelerates, coffee spills on to your lap. On the other hand, when braking from a state of motion the coffee continues forward with the same initial speed and in the same direction, ultimately hitting the windshield or the dashboard. Coffee in motion tends to stay in motion. Have you ever experienced inertia (resisting changes in your state of motion) in a car while braking it to a stop? The force of the road onto the wheels provides the unbalanced force to change the car's state of motion. There is no unbalanced force to change you, the passenger’s state of motion. Thus, you continue to be in motion and slides forward in your seat. A person in motion tends to stay in motion with the same speed and direction ... unless acted upon by the unbalanced force, like the seat belt. Yes..! Seat belts provide safety for passengers whose motions are governed by Newton's laws. It provides the unbalanced force which brings you from a state of motion to a state of rest. Perhaps you could speculate what would happen when no seat belt is used.


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Reading Materials


Modul SCE3105 Phy In Context 22Oct'09\Explaining Motion.pdf (or refer Explaining Motion PDF file) Read the notes on Newton’s First Law of Motion in Page 15 and 16.

Newton’s Second Law of Motion Newton's second law of motion explains the behavior of objects, that all existing forces are not balanced. The second law states that the acceleration of an object is dependent on two variables; the net force acting upon the object and the mass of the object. The acceleration of an object depends directly upon the net force acting on the object, and inversely on the mass of the object. As the force acting on an object is increased, the acceleration of the object is increased. As the mass of an object is increased, the acceleration of the object is decreased. Newton's second law of motion can be formally stated as follows: The acceleration of an object produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object. This verbal statement can be expressed in equation form as follows: a = Fnet / m The above equation is often rearranged to a more familiar form as shown below. The net force is equated to the product of the mass times the acceleration. Fnet = m x a In this entire discussion, the emphasis has been on the net force. The acceleration is directly proportional to the net force; the net force equals mass times acceleration; the acceleration in the same direction as the net force; an acceleration is produced by a net force. It is the net force which is related to acceleration. The net force is the vector sum of all the forces. If all the individual forces acting upon an object are known, then the net force can be determined. Consistent with the above equation, a unit of force is equal to a unit of mass times a unit of acceleration. By substituting standard metric units for force, mass, and acceleration into the above equation, the following unit equivalency can be written.


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The definition of the standard metric unit of force is stated by the above equation. One Newton is defined as the amount of force required to give a 1 kg mass an acceleration of 1 ms-2. This Fnet = m • a equation is often used in algebraic problem-solving. In conclusion, Newton's second law provides the explanation for the behavior of objects in which the forces are not balanced. The law states that unbalanced forces cause objects to accelerate with an acceleration which is directly proportional to the net force and inversely proportional to the mass.

Exercise

A force of 1 Newton applied to a body with a mass of 1 kilogram produces an acceleration of 1 m s-1. How much acceleration do you get if you apply a force of 3 N to a 1 kg body? How much acceleration do you get if you apply a force of 1 N to a 3 kg body?

Newton's Third Law of Motion A force is a push or a pull upon an object due to its interaction with another object. Forces result from interactions! Some forces result from contact interactions (normal, frictional, tensional, and applied forces are examples of contact forces) while other forces are the result of action-at-a-distance interactions (gravitational, electrical, and magnetic forces). According to Newton, whenever objects A and B interact with each other, they exert forces on each other. When you sit on your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces that result from this interaction - the force on the chair and a force on your body. These two forces are called action and reaction forces, are the subjects of Newton's third law of motion. Formally stated, Newton's third law is:

For every action, there is an equal and opposite reaction. The statement above means that for every interaction, there is a pair of forces acting on the two interacting objects. The magnitude of the force acting on the first object is equal to the magnitude of force acting on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs that are equal and opposite, also known as the action-reaction force pairs. A variety of action-reaction force pairs are evident in nature. Consider the movement of a fish through water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the


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water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forward). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for fish to swim. Consider the flying motion of birds. A bird flies by using its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly. Consider the motion of a car on the road. A car is equipped with wheels which spin backwards as shown in Figure 5.3. As the wheels spin backwards, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface. Figure 5.3

Exercise


Modul SCE3105 Phy In Context 22Oct'09\Explaining Motion.pdf (or refer Explaining Motion PDF file)

Read the notes on Newton’s Third Law of Motion and try to find the answer to the problem in Page 5 to 9.

5.3 Application of Newton’s Law of Motion Mass and Weight

A few further comments should be added about the single force which is a source of much confusion to many students of physics - the force of gravity. The force of gravity acting upon an object is sometimes referred to as the weight of the object. Many students of physics confuse weight with mass. The mass of an object refers to the amount of matter contained by the object; the weight of an object is the force of gravity acting upon that object. Mass is related to how much matter is there and weight is related to the gravitaional pull of the Earth (or any other planet) upon that matter.


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The mass of an object (measured in kg) will be the same no matter where in the universe the object is located. Mass is never altered by location, the pull of gravity, speed or even the existence of other forces. For example, a 2-kg object will still have a mass of 2 kg whether it is located on Earth, the moon, or Jupiter; its mass will be 2 kg whether it is moving or not (at least for purposes of our study); and its mass will be 2 kg whether it is being pushed upon or not. On the other hand, the weight of an object (measured in Newtons) will vary according to where in the universe the object is. Weight depends upon which planet is exerting the force and the distance the object is from the planet. Weight, being equivalent to the force of gravity, is dependent upon the value of g. On the earth's surface, g is 9.8 m/s2 (often approximated as 10 m/s2). On the moon's surface, g is 1.7 m/s2. Go to another planet, and there will be another g value. Furthermore, the g value is inversely proportional to the distance from the center of the planet. So if we were to measure g at a distance of 400 km above the earth's surface, then we would find the g value to be less than 9.8 m/s2. Always be cautious of the distinction between mass and weight. It is the source of much confusion for many physics students.

Apparent Weight Consider someone travelling in a lift as shown in Figure 5.4. The forces acting on the person are the weight of the person, Fg and the normal reaction. The weight acted downward due to gravitational force on the person and the normal reaction on the feet of the person from the floor of the lift, FR is due to the person’s weight acting on the floor of the lift. (1) When the lift is stationary or moving with a constant

velocity, these two forces FR and F g must be equal in size and opposite in direction (Newton’s second law of motion). The force acting on the floor of the lift Fon floor by the person’s feet is the same as his weight Fg . Figure 5.4

(2) When the lift accelerates upwards, the net force acting on the person must be non-

zero and act upwards. In order for this to be so, the condition FR > Fg must apply. Notice that, the person’s weight Fg is always the same, the reaction FR and Fon floor will still oppose each other exactly. The person in the lift experience FR on the soles of his feet, as the reaction FR is now greater than his weight, so to him it feels as though he has suddenly got heavier, since we experience our weight as the upward push of whatever we are standing on.

(3) When the lift accelerates downwards, the net force must be non-zero and act

downwards, so that FR < Fg. Now the person feels lighter, as the push of the lift floor on the soles of his feet FR, has decreased. Once more, the person’s weight Fg is always the same and the reaction FR and Fon floor will still oppose each other exactly.

The force FR is the reaction due to the action of the person’s weight acting on the floor Fon

floor and they always opposes each other exactly i.e. FR = Fon floor. FR is known as the apparent weight.


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Supposing that the cable holding the lift breaks. The lift and the person in the lift would experience the same acceleration downwards of g=9.8 ms-2. There will not be any force acting on the floor of the lift as the person in the lift will have no contact with the floor of the lift and the normal reaction FR = 0, i.e his apparent weight would be zero. That is you would be weightless. However, weightlessness does not mean that your weight is zero. Weightlessness means that your apparent weight is zero. For example, astronauts in space have weight but have zero apparent weight so are in a state of weightlessness. Example: Figure 5.5 (a),(b), (c) and (d) show the four different motion of a lift and the readings recorded by the weighing machine indicating the normal reaction or the apparent weight of the person in the lift.

Tutorial 4 (21 hour)

A student stands on a bathroom scale in a lift at rest on the 64th floor of a building. The scale reads 836N. (a) As the lift moves up, the scale reading increases to 936N, then decrease back to 836N. Find the acceleration of the lift.

(b) As the lift approaches the 74th floor, the scale reading drops to 782N. What is the acceleration of the lift?

Frictional Force Push your hand across the table top and feel the force called friction opposing the motion. Friction is often minimised in force and motion problems, but in real world, friction is everywhere. You need it to both start and stop a car. If you have ever walk on ice, you


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know how important friction is. Friction lets a pencil make a mark on paper and an eraser to clear the pencil mark. Friction is the force that opposes motion between two surfaces that are touching each other. The amount of friction between two surfaces depends on two factors – the kinds of surfaces and the force pressing the surfaces together. What happen if we stand still on a slope? The friction between the soles of our shoes and the ground will prevent us from slipping. If the surface is wet or muddy we will begin to slide downwards but friction will still be acting against this movement and reducing its speed. You probably believe that the surface of a highly polished piece of metal is very smooth. If you look under a microscope, you will see that the surface of any object is not smooth. You may view the dips and bumps on the surface as in the Figure 5.6.

Figure 5.6 If two surfaces of solid materials are pressed tightly together, welding, or sticking, occurs in those areas where the highest bumps come into contact with each other. These areas where the bumps stick together are called micro welds and are the source of friction. The larger the force pushing the two surfaces together is, the stronger the micro welds will be, because more of the bumps will come into contact. To break these micro welds and move one surface over the other, you have to apply a force. Friction and Surface Roughness Figure 5.7 shows an illustration of the friction occurring between two surfaces which are influenced by the roughness of the two surfaces.


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Figure 5.7

Can you identify how many types of friction found in everyday life? In everyday life, the effects of friction may be beneficial or harmful to us. To avoid slipping on a slippery ground friction needs to be increased. Can you list the actions that increase friction? Friction can be increased by: Having rougher surfaces – more ‘dips’ and ‘bumps’ Having a greater pressure between the surfaces – this pushes the bumps into each other Changing the nature of the materials involved (e.g. by using rubber between the surfaces) To ensure, the work done is cost effective and efficient, in some cases friction has to be overcome and reduced. Can you list ways that can reduce friction? Friction can be reduced or overcome by: Having smoother surfaces – less bumps to catch; Having less pressure between surfaces – allows the dips and bumps to separate and interact less; Using lubricant such as grease, oil, water or graphite – which stop two surfaces coming into contact by coating each surface with a slippery film; Reducing the area of contact such as by using ball bearing in a bicycle wheel – this reduces the number of bumps that be ‘weld’.


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Reading Materials


Modul SCE3105 Phy In Context 22Oct'09\Explaining Motion.pdf (or refer Explaining Motion PDF file) Read the notes on friction and try to find the answer to the problem in Page 28 to 32 .

Thinking

Access the website below which show an animation on apparent weight of a person in a lift http://glencoe.com/sec/science/physics/ppp_09/animation/Chapter%204/Apparent%20Weight.swf Study the animation carefully and write short notes on the concept of apparent weight based onNewton’s Second Law of motion.

References: Zitzewitz,P.W.(2002) Physics: Principles and Problems. Ohio: Glencoe/McGraw-Hill. (Chapter 6 Forces) http://glencoe.com/sec/science/physics/ppp_09/animation/Chapter%204/Apparent%20Wei

ght.swf (Concept of apparent weight) http://www.physicsclassroom.com/Class/newtlaws/index.cfm (Newton’s Laws of Motion – notes and tutorial)


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TOPIC 7

FORCES IN FLUIDS

Synopsis Matter exists in three different states namely solid, liquid and gas. Gases and liquids are called fluids. One of the characteristics of fluids is that it exerts pressure in all directions. Fluid pressure is very important as it is able to keep a huge ship afloat or keep an enormous aircraft flying. In this topic you will explore a few principles which explain what happen to objects in fluid such as Pascal principle, Archimedes principle and Bernoulli’s principle. In addition, you will be exposed to some of the applications of these principles in our daily life. Learning Outcomes

1. Explain the relationship between density of an object and its buoyancy. 2. Describe Pascal principle and its applications to solve problems relate to brake. 3. Describe Archimedes principle and its applications to solve problem on flotation. 4. Describe Bernoulli’s principle and its applications. 5. Relate all these principles to real life situations.

Overview


Content 7.1 Floating and Sinking Buoyancy How does a boat or ship carrying hundreds of kilograms worth of stuff float when that same stuff would sink to the bottom of the ocean if dumped overboard? What forces act on an object that is placed in fluid?

Forces In Fluids

Float and sink Pascal Principle and its applications

Archimedes Principle and its applications

Bernoulli Principle and its applications


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When a rigid object is submerged in a fluid (completely or partially), there exist an upward force known as buoyant force acting on the object. This buoyant force is the result of

fluid pressure increases with depth

increased pressure is exerted in all directions (Pascal's principle)

an unbalanced upward force on the bottom of a submerged object.

Fluid Pressure Consider that the container in Figure 7.2. The container and its fluid contents are subjected to gravity as an additional force. Thus we must consider that the fluid pressure, ΔP at the bottom of the fluid column of Δh is:

where

- ΔP is the pressure (in Pascal (Pa) in the SI system), or the difference in pressure between the two points (top and bottom) within a fluid column, due to the weight of the fluid;

- ρ is the fluid density (in kilograms per cubic meter in the SI system); - g is acceleration due to gravity (normally using the sea level acceleration due to

Earth's gravity in meters per second squared); - Δh is the height of fluid above the point of measurement of fluid pressure(in meters in

SI). Pascal Principle According to Pascal's Principle, an external pressure applied to any part of an enclosed fluid, is transmitted uniformly to all other parts of the fluid. Pascal’s principle can be interpreted as any change in pressure applied at any given point of the fluid is transmitted undiminished throughout the fluid. (for further detail refer to 7.2 Pascal’s Principle and Its Applications). Unbalanced Upward Force By comparing this upward buoyant force, F2 and its weight, mg you will know whether the object will sink or float. If the buoyant force is greater than its weight, it will float. Conversely, if its weight is greater, it will sink. Consider an imaginery volume of fluid (a cube with each face having an area of A) as shown in Figure 7.3. The sum of all the forces acting on this volume must be zero as it is in equilibrium. There are three vertical forces i.e. the weight of the volume of fluid (mg), the upward force due to pressure from the bottom surface (F2) and the downward force due to pressure on the top surface (F1).

Figure 7.2

Figure 7.3


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F1 = p1 x A F2 = p2 x A (i.e. Force = pressure X area) and

mg = ρ x (y2 – y1)A x g (i.e. Weight of volume of fluid = density of fluid X volume

of cube x acceleration due to gravity)

Since the pressure at the bottom of the object is greater than that at the top of the object, the water exert a net upward force, the buoyant force FB , on the object. The buoyant force FB is equal to the difference in the pressure p2 – p1 times the area A. FB = F2 - F1 = (p2 – p1) A

Since p1 = ρ x g x y1 p2 = ρ x g x y2 ,

therefore FB = ρ x g x (y2 - y1) x A

= ρ x g x V

Where V = volume of the imaginery volume of fluid Buoyant Force Fb is given by the following equation.

Where ρ is the density of the fluid, g is acceleration due to gravity and V is the volume of the object.

Tutorial 5 (

21 hour)

From the above equation, what can you conclude about the buoyant forces acting on object of equal volume?

Reading Materials


Modul SCE3105 Phy In Context 22Oct'09\Topic 7 Forces in fluids.pdf (or refer Topic 7 Forces in fluids PDF file)

Read the notes on Page 1-3 and 6 – 9. Then try to answer the exercises on floating and sinking. 7.2 Archimedes Principle and its Applications Archimedes Principle After completing section 7.1, you should have known that objects of equal volume experienced equal buoyant forces FB, as you have seen that FB depend on V i.e. the volume of the object. The figure shows a block with the same volume as the imaginery volume of fluid in Figure 7.3, will experience

Figure 7.4


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an upward force known as upthrust FB . Similar to the Unbalanced Upward Force explained in 7.1, can you workout how FB is the same as the weight of the fluid displaced by the block, Wliqiud.

This is Archimedes’ Principle, which states that an object immersed either totally or partially in a fluid, has an upward force (upthrust) acting on it equal to the weight of the fluid displaced by the object. The upthrust does not depend on the weight of the object, only on the weight of the displaced fluid. Applications of Archimedes’ Principle .

Figure 7.5

Figure 7.5 shows a toy submarine floating in a bathtub. It's a really fancy sub, made out of steel. The sub weighs one kilogram. When completely submerged, it displaces two kilograms of water. What would you to sink the sub to the bottom of the tub?

Add one kilogram of sand to the sub's interior. Add one kilogram of sand to the sub's interior, plus a little more. Nothing. Since the boat displaces more water than it weighs, it's already on its way down.

After trying the questions above , carry out the following reading activity to find out whether your answered the above questions correctly.

.

Reading Materials

Please press ctrl+click on the pdf file below Modul SCE3105 Phy In Context 22Oct'09\Topic 7 Forces in fluids.pdf (or refer Topic 7 Forces in fluids PDF file). Read the notes on pages 4- 5 and 9–

10. Then try to answer the exercises on Archimedes’ Principle and its applications


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7.3 Pascal’s Principle and its application

Applications of Pascal's Principle According to Pascal’s principle, pressure is transmitted undiminished in an enclosed static fluid. Figure below shows how an external pressure P1 is transmitted from one point (stopper of ajug) to another point (bottom of the jug). Can you figure out how the pressure at the bottom of the jug, P2 is calculated.

Figure 7.3

From the above application of Pascal’s Principle, you will notice how any externally applied pressure is transmitted to all parts of the enclosed fluid, making possible a large multiplication of force (hydraulic press principle). The pressure at the bottom of the jug is equal to the externally applied pressure on the top of the fluid plus the static fluid pressure from the weight of the liquid.

Reading Materials

Please press ctrl+click on the pdf file below Modul SCE3105 Phy In Context 22Oct'09\Topic7 Application Pressure and Fluids.pdf (or refer Topic 7 Application Pressure and Fluids PDF file) Read the notes on Pascal’s Principle and its real life applications in hydraulic press.


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Reading Materials

Access the following website which has an animation on Pascal’s Principle and its applications http://glencoe.com/sec/science/physics/ppp_09/animation/Chapter%2013/Pascals%20Principle.swf

Study the animation carefully and write short notes on the applications of Pascal’s Principle in the animation.

7.4 Bernoulli’s Principle and Its Applications Bernoulli’s Principle The thinker who first formulated the general principles that relate fluids with pressure was a Swiss mathematician and physicist Daniel Bernoulli (1700-1782). Bernoulli is considered the father of fluid mechanics, which is the study of the behavior of gases and liquids at rest and in motion. Hence, he formulated Bernoulli's principle, which states that for all changes in movement, the sum of static and dynamic pressure in a fluid remain the same. A fluid at rest exerts static pressure, which is commonly meant by "pressure," as in "water pressure." As the fluid begins to move, however, a portion of the static pressure—proportional to the speed of the fluid—is converted to what is known as dynamic pressure, or the pressure of movement. In a cylindrical pipe, static pressure is exerted perpendicular to the surface of the container, whereas dynamic pressure is parallel to it. According to Bernoulli's principle, the greater the velocity of flow in a fluid, the greater the dynamic pressure and the lesser the static pressure: in other words, slower-moving fluid exerts greater pressure than faster-moving fluid. The discovery of this principle ultimately made possible the development of the airplane.

The figure below shows how did the wings of airplane experience an upthrust according to Bernoulli’s Principle which state that as the speed of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases.

Figure 7.4


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Bernoulli's principle works on the idea that as a wing passes through the air, its shape make the air travel more over the top of the wing than beneath it (Figure 7.4). This creates a higher pressure beneath the wing than above it. The pressure difference cause the wing to experience an upwards push and lift is created. Bernoulli's principle states that increased air velocity produces decreased pressure.

Lift is produced by an airfoil through a combination of decreased pressure above the airfoil and increased pressure beneath

it.

Reading Materials

Please press ctrl+click on the pdf file below Modul SCE3105 Phy In Context 22Oct'09\Topic 7 Forces in fluids.pdf (or refer Topic 7 Forces in fluids PDF file) Read the notes on Page11–16. Then try to answer the exercises on Bernoulli’s Principle and its applications.

Thinking

Surf the internet and try to solve the crosswords puzzle on forces in fluids. The website below is one of the sources where you can carry out interactive exercises on forces in fluids http://glencoe.com/olc_games/game_engine/content/gln_sci/ppp_09/ch13_w/index.html

References

http://glencoe.com/sec/science/physics/ppp_09/animation/Chapter%2013/Buoyancy.swf (Buoyancy)


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http://www.mhhe.com/physsci/physical/giambattista/buoyancy/buoyancy.html (Interactive tutorial on buoyancy) http://glencoe.com/sec/science/physics/ppp_09/animation/Chapter%2013/Pascals%20Principle.swf http://www.answers.com/topic/pascal-x0027-s-principle http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html (Pascal principle) http://glencoe.com/olc_games/game_engine/content/gln_sci/ppp_09/ch13_w/index.html (Cross word on terminology related to forces in fluids) Zitzewitz,P.W.(2002) Physics: Principles and Problems. Ohio: Glencoe/McGraw-Hill. (Chapter 13 States of Matter)

TOPIC 10

THERMOMETRY AND THERMOMETERS

Synopsis Heat is a form of energy which is vital for our survival. We need heat energy to keep our body warm, to prepare and preserve food as well as to produce many useful things to make our live more comfortable. Temperature is a measure of the amount of heat energy in an object. In this topic, you will be exposed to the concept of temperature and heat energy, temperature scales and various types of thermometer as well as heat transfer and thermal equilibrium. Learning Outcomes

1. Explain the nature of thermal energy. 2. Define temperature and distinguish it from thermal energy. 3. Use the Celsius and Kelvin temperature scales and convert from one scale to

another. 4. Define specific heat and calculate heat transfer.

Overview


Thermometry dan thermometers

Temperature dan thermal energy

Thermal Equilibrium

Types of Thermometer

Heat Transfer


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Content 10.1 Temperature dan thermal energy

According to kinetic molecular theory, matter is made up of many tiny particles that are always in motion. The temperature of an object depends on the kinetic energy of these particles. In a hot body, the particles move faster and thus have greater kinetic energy than particles in a cooler body. The overall energy of motion of the particles that made up an object is called the thermal energy of that object. Thermal energy depends on the number of particles in the object.

Hotness is a property of an object called its temperature. Temperature of an object only depends on the average kinetic energy of the particles in the object. The average kinetic energy is the total kinetic energy of the all the particles in the object divided by the total number of particles. Therefore unlike thermal energy, temperature is independent of the number of particles in the object.

Thermal energy will always flow from a hotter object to a cooler object.

Making Notes

To foster better understanding of kinetic molecular theory and thermal energy of particles, please read the following website: http://www.saburchill.com/physics/chapters/0098.html

and Chapter 12 Thermal Energy of Principles and Problems Glencoe/McGraw-Hill Publication. (Pg 274) Make short notes related to the two topics.

10.2 Equilibrium and Thermometry

Thermal Equilibrium.

When two objects of different temperatures come in contact, the hotter object with higher temperature (red coloured box) becomes cooler while a cooler object with a lower temperature (blue coloured box) becomes warmer.

Figure 10.2


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After some time, there is no more change in hotness, and to our touch, they feel the same. When the heat flow or thermal changes between two objects in contact stopped, we say that the two objects are in thermal equilibrium. We can then say that the hotness or temperature of the objects in contact is the same when they are in thermal equilibrium.

To foster better understanding of thermal equilibrium, please read the following website:

http://physics.about.com/od/thermodynamics/p/thermodynamics.htm and Chapter 12 Thermal Energy of Principles and Problems Glencoe/McGraw-Hill Publication.(Pg 275) Make short notes related to the topics. Figure 10.3 shows a thermometer (object C) which is in thermal equilibrium with two objects A and B separately. The two objects A and B which are not in contact are also in thermal equilibrium and have the same temperature.

Thermometry Thermometry is the science of measuring the temperature of a system or the ability of a system to transfer heat to another system.

Thermometers is a device that measures temperature. The operation of

thermometer depends on some property of materials that change in some way when they are heated or cooled. For example, in a mercury or alcohol thermometer the liquid expands as it is heated and contracts when it is cooled, so the length of the liquid column is longer or shorter depending on the temperature. Modern thermometers are calibrated in standard temperature units such as Fahrenheit or Celsius.

Tutorial 6 (

21 hour)


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10.3 Types of Thermometer

Physical property of materials that change in some way when they are heated or cooled is known as thermometric property of the material. Besides mercury or alcohol thermometer, there are other types of thermometer which operate based on other thermometric property such as gas pressure, electromotive force(e.m.f.), electric resistance etc. Below is a list of thermometers. Try to find the thermometric property for each of the thermometer. • Beckmann differential thermometer • Bi-metal mechanical thermometer • Electrical resistance thermometer • Galileo thermometer • Infrared thermometer • Liquid Crystal Thermometer • Medical thermometer (e.g. oral thermometer, rectal thermometer, basal thermometer) • Reversing thermometer • Silicon bandgap temperature sensor • Six's thermometer- also known as a Maximum minimum thermometer • Thermistor • Thermocouple • Coulomb blockade thermometer Temperature scales were developed by scientists to measure and compare temperatures. A scale based on the property of water was devised in 1741 by a Swedish astronomer and physicist Anders Celsius. On this scale is now known as Celsius scale. The freezing point of pure water is 0 C and the boiling point of pure water at sea level is 100 C.

Do you know what is the temperature scale used for S.I. unit? The temperature scale for S.I. unit is based on ideal gas. Ideal gas when cooled will contract to only its molecular size (i.e. volume of ideal gas is now zero) at -273.15 C. At this temperature, all the gas molecules has zero thermal energy and the temperature of the ideal gas cannot be reduced anymore. As such, -273.15 C is known as the absolute zero. The S.I. unit for temperature, known as Kelvin temperature scale is based on absolute zero. In Kelvin temperature scale, its zero point start from absolute zero. Each division in Kelvin temperature scale is similar to that of Celsius temperature scale.

Can you work out what is the equivalent temperature on the Kelvin scale for the freezing point and boiling point of water?


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Making Notes

To foster better understanding of thermometer, temperature scale and measurement, please read the following website:

http://www.saburchill.com/physics/chapters/0097.html http://physics.about.com/od/glossary/g/temperature.htm

and Chapter 12 Thermal Energy of Principles and Problems Glencoe/McGraw-Hill Publication.(Pg 276 & 277)

Make short notes related to the above topics.

Figure 10.4 shows three different temperature scales. Can you determine the freezing point and boiling point for water on each scale?

Figure 10.4 Do the following exercises on temperature conversions: a) A temperature of -35°C is equal to °F. b) A temperature of 54°F is equal to °C. c) A temperature of 300°K is equal to °C d) A temperature of 200°F is equal to °K e) A temperature of 64°C is equal to °K f) A temperature of 100°K is equal to °F

Exercise


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Thermal Energy Transfer

The specific heat of an object S is defined in the following way: Take an object of mass m , put in Q amount of heat and carefully note the temperature rise, then S is given by

In this definition mass is usually in either grams or kilograms and temperature is either in Kelvin or degrees Celsius. Note that the specific heat is "per unit mass".

A related quantity is called the heat capacity (C). of an object. The relation between S and C is C = (mass of object) x (specific heat of object).

Example 1: How much energy does it take to raise the temperature of 50 g of copper by

10 0C? (Specific heat of copper = 0.385 Jg-1 0C-1)

Example 2: If we add 30 J of heat to 10 g of aluminium, by how much will its temperature

increase? (Specific heat of copper = 0.902 Jg-1 0C-1)

Thus, if the initial temperature of the aluminium was 20 0C then after the heat is added the temperature will be 23.3 0C.

Heat transfers from one object to another by one of these three mechanisms

• Conduction

• Convention, and

• Radiation


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Conduction

Regions of greater molecular kinetic energy will pass their thermal energy to regions with less molecular energy through direct molecular collisions. In the atmosphere, heating by conduction occurs near the ground surface, where air molecules are warmed by directly contacting the surface.

Energy is transferred through molecular interactions. They collide losing some of their energy to less energetic molecules in a cooler part of the object.

Convection

Liquid and gases are poor thermal conductors but if they are free to circulate, they can carry thermal energy from one place to another very quickly. For example, warm air rises when it is replaced by cooler, denser air sinking around it. Heat transfer as a result of mass transfer in liquids and gases is known as convection.

Free convection happened when there is heat-induced fluid motion in initially static fluids (liquid or gas). If the static fluid is heated, it loses density and rises. If cooled, it will become dense and sinks. It is a gravity gradient that induces motion through buoyancy.

Forced convection happened when the fluid is already in motion, heat conducted into the fluid will be transported away mainly by fluid convection. Pressure gradient forces drive the fluid motion. The fiqures below show convection in air and water.

Radiation Radiation is the transfer of heat by electromagnetic waves. Thermal energy is radiated at wavelengths determined by the temperature of the surface. For example, short wavelengths for the sun and long wavelengths for sun-warmed materials such as brick.

Do you know why in hot, sunny countries, houses are painted white? Bright, shiny materials reflect radiation while dull, black materials absorb it. So to keep the


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houses cool inside, houses are painted white in hot countries such as Malaysia. Good emitters of thermal radiation are also good radiators. White or silvery surfaces are poor absorbers because they reflect most of the thermal radiation away. All objects with a temperature greater than 0 K emit small amounts of electromagnetic radiation (radiant energy). Electromagnetic radiation is transmitted through empty space at 3 x 10 8 m s 1 .

Tutorial 7(

21 hour)

Study the figure below, can you tell A, B and C stand for which type of heat transfer?


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Making Notes

To foster better understanding of heat transfer, please read the following website:

http://physics.about.com/od/thermodynamics/p/thermodynamics.htm http://physics.about.com/od/thermodynamics/f/heattransfer.htm

Make short notes related to the above topics.

Exercise

Surf the internet and try to answer questions on thermal energy. The website below is one of the sources where you can carry out interactive exercises on thermal energy http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter12/interactive_tutor.html

References http://www.saburchill.com/physics/chapters/0097.html (Celsius Temperature scale) http://www.saburchill.com/physics/chapters/0098.html (Kinetic theory of matter) http://physics.about.com/od/thermodynamics/p/thermodynamics.htm (Thermal equilibrium and heat transfer) http://physics.about.com/od/glossary/g/temperature.htm (Thermometry, temperature and temperature scale) http://physics.about.com/od/thermodynamics/p/thermodynamics.htm http://physics.about.com/od/thermodynamics/f/heattransfer.htm (Methods of heat transfer) http://glencoe.mcgraw-hill.com/sites/0078807220/student_view0/chapter12/interactive_tutor.html (Thermal energy) Zitzewitz,P.W.(2002) Physics: Principles and Problems. Ohio: Glencoe/McGraw-Hill. (Chapter 12 Thermal Energy)


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TOPIC 11

USING LIGHT

Synopsis: We are able to see objects because light is reflected or emitted by it. Light is emitted by various sources such as florescence light, television or LED, but the main source of light is the sun. Light from the sun can be reflected by mirrors, white surfaces, moon, trees and also by other coloured objects. We are able to see an object because light travels from the object to our eyes. In the study of light, light rays are represented by straight narrow rays. The study of light in this topic investigates the reflection and refraction phenomenon. Learning Outcomes: 1. Discuss reflection and mirrors. 2. Discuss refraction and lenses. 3. Discuss the structure and working principle of a microscope and a telescope. Overview

Figure 11.1 11.1 Reflection and mirrors Law of reflection : The reflected rays by a smooth surface are evenly distributed; where else the reflected rays by a rough surface are uneven and diverge.

Light

Reflection Refraction

Mirrors Lenses

Microscope

Telescope

Concave mirror

Convex mirror


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The basis for the law of reflection are (Figure 11.2): - Incident ray, i is equal to the reflected ray, r. - The incident, reflected ray and the normal line are on the same plane.

Figure 11.2

Plane mirrors

Mirrors are able to form image because light from the object reach the mirror and being reflected to our eyes as shown in Figure 11.3.

Rajah 11.3

Exercise

mirror

i

r normal line

Test your understanding by trying out the interactive tutorial at the following webpage: http://glencoe.com/olc_games/game_engine/content/gln_sci/ppp_09/ch17/ch17_1/index.html


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Curved mirrors: There are two types of curved mirrors; concave mirror and convex mirror. The centre of curvature, C of a mirror is the centre of the sphere where the mirror is made up of. The radius of curvature, r is the distance between the centre of the sphere and the surface of the mirror. When parallel light rays fall on the surface of a concave mirror, the rays will be focused to a point (Figure 11.4a). Light rays from the object (AB) will reach the mirror and reflected back and form image as follows: (a) Parallel light from the object will be reflected to the focal point, F (b) Light rays that pass through the focal point will be reflected parallel to the parallel axis. (c) Light rays that pass through the center of curvature, C will be reflected back to C. The intersection of these light rays forms the image, A1B1 which is real, inverted and smaller.

(a) (b)

Figure 11.4 The same rules apply for the formation of image by a convex mirror. When parallel light rays falls on a convex mirror, the rays will diverge as shown in Figure 11.4b. Thus the image form is virtual, smaller and upright. The characteristics of image formed by reflection by curved mirrors are dependent on object’s distance, u from the mirror. Try out the tutorial below to find out the characteristics of images formed when object is placed at different positions from concave and convex mirrors.

Tutorial 8 (

21 hour)

Find information on how to draw a ray diagram for various positions of the object, u from concave and convex mirrors. State the characteristics of images formed. Use the link below to help you in your search. http://en.wikipedia.org/wiki/Convex_mirror Using Light.doc


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11.2 Refraction and lenses Refraction is the bending of a light ray as it passes obliquely from one medium to another.

Rajah 11.5

The refractive index, n can be defined as: n = speed of light in vacuum or air, c speed of light in medium, v n = sin i sin r (i = incident angle, r = reflective angle) n = Ho

H1 ( Ho = depth of object in water, HI = depth of image in water) The value of refractive index changes with different materials as shown in Table 11.1.

Material Refractive index, n Air 1.00 Water 1.33 Perspex 1.49 Glass 1.48-1.96 Diamond 2.42

Table 11.1 Convex and concave lenses Parallel light entering a convex lens will be refracted to a point called the focal point as shown in Figure 11.6 .

Figure 11.6


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Parallel light entering the lens will be refracted and diverge away after passing through the lens.

Figure 11.7

Reading Materials

Please press ctrl+click on the pdf file below Modul SCE3105 Phy In Context 22Oct'09\Topic 11 Using Light.pdf (or refer Topic 11 Using Light PDF file)

Read the notes on Formula for mirror and lenses on page 11 to 14.

Tutorial 9 (

21 hour)

Find information on ray diagrams of convex and concave lenses for various positions of object dan state the characteristics of image at these positions.The following webpage could be of help: http://www.glenbrook.k12.il.us/GBSSCI/PHYS/class/refrn/u14l5da.html


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11.3 Microscope dan Telescope Lenses could be used to produce optical instruments such as compound microscope and astronomical telescope. These instruments require the arrangement of two lenses with different powers. Ray diagram of a compound microscope

Figure 11.8

Ray diagram of an astronomical telescope

Figure 11.9

Thinking

Think how the arrangement two convex lenses of different powers could produce compound microscope and astronomical telescope. Get information from the link below: http://www.saburchill.com/physics/chapters3/0018.html http://physics.bu.edu/~duffy/PY106/Instruments.html


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References: http://en.wikipedia.org/wiki/Convex_mirror (Ray diagrams) http://www.glenbrook.k12.il.us/GBSSCI/PHYS/class/refrn/u14l5da.html (lenses)

http://www.saburchill.com/physics/chapters3/0018.html http://physics.bu.edu/~duffy/PY106/Instruments.html (Mikroskope dan telescope) http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=16.0 (Reflection and refraction –light wave animation) Zitzewitz,P.W.(2002) Physics: Principles and Problems. Ohio: Glencoe/McGraw-Hill. (Chapter 18 Mirrors and Lenses)

Pressure and Fluids

In terms of physics, both gases and liquids are referred to as fluids—that is, substances that conform to the shape of their container. Air pressure and water pressure are thus specific subjects under the larger heading of "fluid pressure." A fluid responds to pressure quite differently than a solid does. The density of a solid makes it resistant to small applications of pressure, but if the pressure increases, it experiences tension and, ultimately, deformation. In the case of a fluid, however, stress causes it to flow rather than to deform.

There are three significant characteristics of the pressure exerted on fluids by a container. First of all, a fluid in a container experiencing no external motion exerts a force perpendicular to the walls of the container. Likewise, the container walls exert a force on the fluid, and in both cases, the force is always perpendicular to the walls.

In each of these three characteristics, it is assumed that the container is finite: in other words, the fluid has nowhere else to go. Hence, the second statement: the external pressure exerted on the fluid is transmitted uniformly. Note that the preceding statement was qualified by the term "external": the fluid itself exerts pressure whose force component is equal to its weight. Therefore, the fluid on the bottom has much greater pressure than the fluid on the top, due to the weight of the fluid above it.

Third, the pressure on any small surface of the fluid is the same, regardless of that surface's orientation. In other words, an area of fluid perpendicular to the container walls experiences the same pressure as one parallel or at an angle to the walls. This may seem to contradict the first principle, that the force is perpendicular to the walls of the container. In fact, force is a vector quantity, meaning that it has both magnitude and direction, whereas pressure is a scalar, meaning that it has magnitude but no specific direction.

Real-Life Applications

Pascal's Principle and the Hydraulic Press

The three characteristics of fluid pressure described above have a number of implications and applications, among them, what is known as Pascal's principle. Like the SI unit of pressure, Pascal's principle is named after Blaise Pascal (1623-1662), a French mathematician and physicist who formulated the second of the three statements: that the external pressure applied on a fluid is transmitted uniformly throughout the entire body of that fluid. Pascal's principle became the basis for one of the important machines ever developed, the hydraulic press.

A simple hydraulic press of the variety used to raise a car in an auto shop typically consists of two large cylinders side by side. Each cylinder contains a piston, and the cylinders are connected at the bottom by a channel containing fluid. Valves control flow between the two cylinders. When one applies force by pressing down the piston in one

cylinder (the input cylinder), this yields a uniform pressure that causes output in the second cylinder, pushing up a piston that raises the car.

In accordance with Pascal's principle, the pressure throughout the hydraulic press is the same, and will always be equal to the ratio between force and pressure. As long as that ratio is the same, the values of F and A may vary. In the case of an auto-shop car jack, the input cylinder has a relatively small surface area, and thus, the amount of force that must be applied is relatively small as well. The output cylinder has a relatively large surface area, and therefore, exerts a relatively large force to lift the car. This, combined with the height differential between the two cylinders (discussed in the context of mechanical advantage elsewhere in this book), makes it possible to lift a heavy automobile with a relatively small amount of effort.

The Hydraulic Ram

The car jack is a simple model of the hydraulic press in operation, but in fact, Pascal's principle has many more applications. Among these is the hydraulic ram, used in machines ranging from bulldozers to the hydraulic lifts used by firefighters and utility workers to reach heights. In a hydraulic ram, however, the characteristics of the input and output cylinders are reversed from those of a car jack.

The input cylinder, called the master cylinder, has a large surface area, whereas the output cylinder (called the slave cylinder) has a small surface area. In addition—though again, this is a factor related to mechanical advantage rather than pressure, per se—the master cylinder is short, whereas the slave cylinder is tall. Owing to the larger surface area of the master cylinder compared to that of the slave cylinder, the hydraulic ram is not considered efficient in terms of mechanical advantage: in other words, the force input is much greater than the force output.

Nonetheless, the hydraulic ram is as well-suited to its purpose as a car jack. Whereas the jack is made for lifting a heavy automobile through a short vertical distance, the hydraulic ram carries a much lighter cargo (usually just one person) through a much greater vertical range—to the top of a tree or building, for instance.

Exploiting Pressure Differences

Pumps

A pump utilizes Pascal's principle, but instead of holding fluid in a single container, a pump allows the fluid to escape. Specifically, the pump utilizes a pressure difference, causing the fluid to move from an area of higher pressure to one of lower pressure. A very simple example of this is a siphon hose, used to draw petroleum from a car's gas tank. Sucking on one end of the hose creates an area of low pressure compared to the relatively high-pressure area of the gas tank. Eventually, the gasoline will come out of the low-pressure end of the hose. (And with luck, the person siphoning will be able to anticipate this, so that he does not get a mouthful of gasoline!)

The piston pump, more complex, but still fairly basic, consists of a vertical cylinder along which a piston rises and falls. Near the bottom of the cylinder are two valves, an inlet valve through which fluid flows into the cylinder, and an outlet valve through which fluid flows out of it. On the suction stroke, as the piston moves upward, the inlet valve opens and allows fluid to enter the cylinder. On the downstroke, the inlet valve closes while the outlet valve opens, and the pressure provided by the piston on the fluid forces it through the outlet valve.

One of the most obvious applications of the piston pump is in the engine of an automobile. In this case, of course, the fluid being pumped is gasoline, which pushes the pistons by providing a series of controlled explosions created by the spark plug's ignition of the gas. In another variety of piston pump—the kind used to inflate a basketball or a bicycle tire—air is the fluid being pumped. Then there is a pump for water, which pumps drinking water from the ground It may also be used to remove desirable water from an area where it is a hindrance, for instance, in the bottom of a boat.

Bernoulli's Principle

Though Pascal provided valuable understanding with regard to the use of pressure for performing work, the thinker who first formulated general principles regarding the relationship between fluids and pressure was the Swiss mathematician and physicist Daniel Bernoulli (1700-1782). Bernoulli is considered the father of fluid mechanics, the study of the behavior of gases and liquids at rest and in motion.

While conducting experiments with liquids, Bernoulli observed that when the diameter of a pipe is reduced, the water flows faster. This suggested to him that some force must be acting upon the water, a force that he reasoned must arise from differences in pressure. Specifically, the slower-moving fluid in the wider area of pipe had a greater pressure than the portion of the fluid moving through the narrower part of the pipe. As a result, he concluded that pressure and velocity are inversely related—in other words, as one increases, the other decreases.

Hence, he formulated Bernoulli's principle, which states that for all changes in movement, the sum of static and dynamic pressure in a fluid remain the same. A fluid at rest exerts static pressure, which is commonly meant by "pressure," as in "water pressure." As the fluid begins to move, however, a portion of the static pressure—proportional to the speed of the fluid—is converted to what is known as dynamic pressure, or the pressure of movement. In a cylindrical pipe, static pressure is exerted perpendicular to the surface of the container, whereas dynamic pressure is parallel to it.

According to Bernoulli's principle, the greater the velocity of flow in a fluid, the greater the dynamic pressure and the less the static pressure: in other words, slower-moving fluid exerts greater pressure than faster-moving fluid. The discovery of this principle ultimately made possible the development of the airplane.

As fluid moves from a wider pipe to a narrower one, the volume of that fluid that moves a given distance in a given time period does not change. But since the width of the narrower pipe is smaller, the fluid must move faster (that is, with greater dynamic pressure) in order to move the same amount of fluid the same distance in the same amount of time. One way to illustrate this is to observe the behavior of a river: in a wide, unconstricted region, it flows slowly, but if its flow is narrowed by canyon walls, then it speeds up dramatically.

Bernoulli's principle ultimately became the basis for the airfoil, the design of an airplane's wing when seen from the end. An airfoil is shaped like an asymmetrical teardrop laid on its side, with the "fat" end toward the airflow. As air hits the front of the airfoil, the airstream divides, part of it passing over the wing and part passing under. The upper surface of the airfoil is curved, however, whereas the lower surface is much straighter.

As a result, the air flowing over the top has a greater distance to cover than the air flowing under the wing. Since fluids have a tendency to compensate for all objects with which they come into contact, the air at the top will flow faster to meet with air at the bottom at the rear end of the wing. Faster airflow, as demonstrated by Bernoulli, indicates lower pressure, meaning that the pressure on the bottom of the wing keeps the airplane aloft.

Buoyancy and Pressure

One hundred and twenty years before the first successful airplane flight by the Wright brothers in 1903, another pair of brothers—the Mont-golfiers of France—developed another means of flight. This was the balloon, which relied on an entirely different principle to get off the ground: buoyancy, or the tendency of an object immersed in a fluid to float. As with Bernoulli's principle, however, the concept of buoyancy is related to pressure.

In the third century B.C., the Greek mathematician, physicist, and inventor Archimedes (c. 287-212 B.C.) discovered what came to be known as Archimedes's principle, which holds that the buoyant force of an object immersed in fluid is equal to the weight of the fluid displaced by the object. This is the reason why ships float: because the buoyant, or lifting, force of them is less than equal to the weight of the water they displace.

The hull of a ship is designed to displace or move a quantity of water whose weight is greater than that of the vessel itself. The weight of the displaced water—that is, its mass multiplied by the downward acceleration caused by gravity—is equal to the buoyant force that the ocean exerts on the ship. If the ship weighs less than the water it displaces, it will float; but if it weighs more, it will sink.

The factors involved in Archimedes's principle depend on density, gravity, and depth rather than pressure. However, the greater the depth within a fluid, the greater the pressure that pushes against an object immersed in the fluid. Moreover, the overall

pressure at a given depth in a fluid is related in part to both density and gravity, components of buoyant force.

Pressure and Depth

The pressure that a fluid exerts on the bottom of its container is equal to dgh, where d is density, g the acceleration due to gravity, and h the depth of the container. For any portion of the fluid, h is equal to its depth within the container, meaning that the deeper one goes, the greater the pressure. Furthermore, the total pressure within the fluid is equal to dgh + pexternal, where pexternal is the pressure exerted on the surface of the fluid. In a piston-and-cylinder assembly, this pressure comes from the piston, but in water, the pressure comes from the atmosphere.

In this context, the ocean may be viewed as a type of "container." At its surface, the air exerts downward pressure equal to 1 atm. The density of the water itself is uniform, as is the downward acceleration due to gravity; the only variable, then, is h, or the distance below the surface. At the deepest reaches of the ocean, the pressure is incredibly great—far more than any human being could endure. This vast amount of pressure pushes upward, resisting the downward pressure of objects on its surface. At the same time, if a boat's weight is dispersed properly along its hull, the ship maximizes area and minimizes force, thus exerting a downward pressure on the surface of the water that is less than the upward pressure of the water itself. Hence, it floats.

Pressure and the Human Body

Air Pressure

The Montgolfiers used the principle of buoyancy not to float on the water, but to float in the sky with a craft lighter than air. The particulars of this achievement are discussed elsewhere, in the context of buoyancy; but the topic of lighter-than-air flight suggests another concept that has been alluded to several times throughout this essay: air pressure.

Just as water pressure is greatest at the bottom of the ocean, air pressure is greatest at the surface of the Earth—which, in fact, is at the bottom of an "ocean" of air. Both air and water pressure are examples of hydrostatic pressure—the pressure that exists at any place in a body of fluid due to the weight of the fluid above. In the case of air pressure, air is pulled downward by the force of Earth's gravitation, and air along the surface has greater pressure due to the weight (a function of gravity) of the air above it. At great heights above Earth's surface, however, the gravitational force is diminished, and, thus, the air pressure is much smaller.

In ordinary experience, a person's body is subjected to an impressive amount of pressure. Given the value of atmospheric pressure discussed earlier, if one holds out one's hand—assuming that the surface is about 20 in2 (0.129 m2)—the force of the air resting on it is nearly 300 lb (136 kg)! How is it, then, that one's hand is not crushed by all this weight? The reason is that the human body itself is under pressure, and that the interior of the body exerts a pressure equal to that of the air.

The Response to Changes in Air Pressure

The human body is, in fact, suited to the normal air pressure of 1 atm, and if that external pressure is altered, the body undergoes changes that may be harmful or even fatal. A minor example of this is the "popping" in the ears that occurs when one drives through the mountains or rides in an airplane. With changes in altitude come changes in pressure, and thus, the pressure in the ears changes as well.

As noted earlier, at higher altitudes, the air pressure is diminished, which makes it harder to breathe. Because air is a gas, its molecules have a tendency to be non-attractive: in other words, when the pressure is low, they tend to move away from one another, and the result is that a person at a high altitude has difficulty getting enough air into his or her lungs. Runners competing in the 1968 Olympics at Mexico City, a town in the mountains, had to train in high-altitude environments so that they would be able to breathe during competition. For baseball teams competing in Denver, Colorado (known as "the Mile-High City"), this disadvantage in breathing is compensated by the fact that lowered pressure and resistance allows a baseball to move more easily through the air.

If a person is raised in such a high-altitude environment, of course, he or she becomes used to breathing under low air pressure conditions. In the Peruvian Andes, for instance, people spend their whole lives at a height more than twice as great as that of Denver, but a person from a low-altitude area should visit such a locale only after taking precautions. At extremely great heights, of course, no human can breathe: hence airplane cabins are pressurized. Most planes are equipped with oxygen masks, which fall from the ceiling if the interior of the cabin experiences a pressure drop. Without these masks, everyone in the cabin would die.

Blood Pressure

Another aspect of pressure and the human body is blood pressure. Just as 20/20 vision is ideal, doctors recommend a target blood pressure of "120 over 80"—but what does that mean? When a person's blood pressure is measured, an inflatable cuff is wrapped around the upper arm at the same level as the heart. At the same time, a stethoscope is placed along an artery in the lower arm to monitor the sound of the blood flow. The cuff is inflated to stop the blood flow, then the pressure is released until the blood just begins flowing again, producing a gurgling sound in the stethoscope.

The pressure required to stop the blood flow is known as the systolic pressure, which is equal to the maximum pressure produced by the heart. After the pressure on the cuff is reduced until the blood begins flowing normally—which is reflected by the cessation of the gurgling sound in the stethoscope—the pressure of the artery is measured again. This is the diastolic pressure, or the pressure that exists within the artery between strokes of the heart. For a healthy person, systolic pressure should be 120 torr, and diastolic pressure 80 torr.

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