Mass & Weight Friction Introduction Section 0 Lecture 1 Slide 1 Lecture 8-9 Slide 1 INTRODUCTION TO...

Mass & Weight Friction

Introduction Section 0 Lecture 1 Slide 1

Lecture 8-9 Slide 1

INTRODUCTION TO Modern Physics PHYX 2710

Fall 2004

Physics of Technology—PHYS 1800

Spring 2009

Physics of Technology

PHYS 1800

Lecture 8

Mass & WeightMotion with Friction



Lecture 8-9 Slide 2


Fall 2004


Spring 2009

PHYSICS OF TECHNOLOGY Spring 2009 Assignment Sheet

*Homework Handout

Date Day Lecture Chapter Homework Due Jan 5 6 7 9

M T W F*

Class Admin: Intro.Physics Phenomena Problem solving and math Units, Scalars, Vectors, Speed and Velocity

1 App. B, C 1 2

-

Jan 12 14 16

M W F*

Acceleration Free Falling Objects Projectile Motion

2 3 3

1

Jan 19 21 23

M W F*

Martin Luther King Newton’s Laws Mass and Weight

No Class 4 4

2

Jan 26 28 29 30

M W Th F

Motion with Friction Review Test 1 Circular Motion

4 1-4 1-4 5

3

Feb 2 4 6

M W F*

Planetary Motion and Gravity Energy Harmonic Motion

5 6 6

4

Feb 9 11 13

M W F*

Momentum Impulse and Collisions Rotational Motion

7 7 8

5

Feb 16 17 18 19 20

M Tu W H F*

Presidents Day Angular Momentum (Virtual Monday) Review Test 2 Static Fluids, Pressure

No Class 8 5-8 5-8 9

-

Feb 23 25 27

M W F*

Flotation Fluids in Motion Temperature and Heat

9 9 10

6

Mar 2 4 6

M W F*

First Law of Thermodynamics Heat flow and Greenhouse Effect Climate Change

10 10 -

7

Mar 9-13 M-F Spring Break No Classes Mar 16 18 20

M W F*

Heat Engines Power and Refrigeration Electric Charge

11 11 12

8

Mar 23 25 26 27

M W H F*

Electric Fields and Electric Potential Review Test 3 Electric Circuits

12 13 9-12 13

-

Mar 30 Apr 1 3

M W F

Magnetic Force Review Electromagnets Motors and Generators

14 9-12 14

9

Apr 6 8 10

M W F*

Making Waves Sound Waves E-M Waves, Light and Color

15 15 16

10

Apr 13 15 17

M W F*

Mirrors and Reflections Refraction and Lenses Telescopes and Microscopes

17 17 17

11

Apr 20 22 24

M W F

Review Seeing Atoms The really BIG & the really small

1-17 18 (not on test) 21 (not on test)

No test week 12

May 1 F Final Exam: 09:30-11:20am



Lecture 8-9 Slide 3


Fall 2004


Spring 2009


PHYS 1800

Lecture 8


Introduction and Review



Lecture 8-9 Slide 4


Fall 2004


Spring 2009

Dennison’s Laws of Motion

1. Stuff happens (or not).

2. The bigger they are the harder they fall.

3. You get what you give.



Lecture 8-9 Slide 5


Fall 2004


Spring 2009

Newton’s Laws in Review

• 1st Law —a special case of the 2nd Law for statics, with a=0 or Fnet=0 • An objects velocity remains unchanged, unless

a force acts on the object.

• 2nd Law (and 1st Law)—How motion of a object is effected by a force.– The acceleration of an object is directly

proportional to the magnitude of the imposed force and inversely proportional to the mass of the object. The acceleration is the same direction as that of the imposed force.

• 3rd Law —Forces come from interactions with other objects.• For every action (force), there is an equal but

opposite reaction (force).

F ma

units : 1 newton = 1 N = 1 kgm s2



Lecture 8-9 Slide 6


Fall 2004


Spring 2009

Newton’s First Law of Motion

An object remains at rest, or in uniform motion in a straight line, unless it is compelled to change by an externally imposed force.



Lecture 8-9 Slide 7


Fall 2004


Spring 2009

Newton’s Second Law of Motion

The acceleration of an object is directly proportional to the magnitude of the imposed force and inversely proportional to the mass of the object.

The acceleration is the same direction as that of the imposed force.



Lecture 8-9 Slide 8


Fall 2004


Spring 2009

Newton’s Second Law of Motion

Note that a force is proportional to an object’s acceleration, not its velocity.

Precise definitions of some commonly used terms:

The mass of an object is a quantity that tells us how much resistance the object has to a change in its motion. This resistance to a change in motion is called inertia.

F ma

units : 1 newton = 1 N = 1 kgm s2

Force has dimensions of (MLT-2)



Lecture 8-9 Slide 9


Fall 2004


Spring 2009


PHYS 1800

Lecture 8


Mass & Weight



Lecture 8-9 Slide 10


Fall 2004


Spring 2009

Mass and Weight

• What exactly is mass?

• Is there a difference between mass and weight?

• If something is weightless in space, does it still have mass?





Fall 2004


Spring 2009

Mass, Weight, and Inertia

• A much larger force is required to produce the same acceleration for the larger mass.

• Inertia is an object’s resistance to a change in its motion.

• Mass is a measure of an object’s inertia.

• The units of mass are kilograms (kg).





Fall 2004


Spring 2009


• An object’s weight is the gravitational force acting on the object.

• Weight is a force, measured in units of newtons (N).

• In the absence of gravity, an object has no weight but still has the same mass.





Fall 2004


Spring 2009

Mass Standard

The International Prototype Kilogram (“IPK”) is the kilogram.

It sits next to an inch-based ruler for scale. The IPK is made of a platinum-iridium alloy and is stored in a vault at the BIPM in Sèvres, France.

How do we determine what the magnitude of a gravitational force (weight) is?





Fall 2004


Spring 2009


• Objects of different mass experience the same gravitational acceleration on Earth: g = 9.8 m/s2

• By Newton’s 2nd Law, F = ma, the weight is W = mg.

• Different gravitational forces (weights) act on falling objects of different masses, but the objects have the same acceleration.





Fall 2004


Spring 2009

Weight

Example: A body of mass 100 kg on the moon weighs:

W = m . g = 100 x 9.8 / 6 = 163 N

Note: In English (imperial) system, weight (W) is measured in lbs (pounds) – which is also a force.

1 lbs = 4.45 N

A mass of 1 kg therefore weighs 2.2 lbs near Earth’s surface (or 9.8 N).

(Compared with 980 N on Earth)

so, W = 163 N = 36.6 lbs only!





Fall 2004


Spring 2009


PHYS 1800

Lecture 8


A closer Look at Forces





Fall 2004


Spring 2009

A Closer Look at the Table

• The forces acting on the book are W (gravitational force from Earth) and N (normal force from table).

• Normal force refers to the perpendicular force a surface exerts on an object.





Fall 2004


Spring 2009

An uncompressed spring and the same spring supporting a book. The compressed spring exerts an upward force on the book.

Third-Law Action/Reaction Pair





Fall 2004


Spring 2009

Bonds between atoms in a compressed solid can be treated as compressed springs.

Ultimately the forces come from electrostatic interactions between electrons and protons (and a little quantum mechanics).

Compression on an Atomic Scale

Fspring=-k Δx++

+

++

+

++

+





Fall 2004


Spring 2009

Third-Law Action/Reaction Pair

The car pushes against the road, and the road, in turn, pushes against the car.





Fall 2004


Spring 2009

What is friction?• A resistive force opposing motion.

• So far we have assumed many examples with no friction but friction is a very important force in our lives…

• No surface is perfectly smooth when viewed at the atomic level!

• Frictional forces arise between two surfaces in contact because they tend to dig into each other.

contact points

Two objects in contact supported by a few high spots or “prominences”.





Fall 2004


Spring 2009

What is friction?

• Friction is known to be independent of surface area – counter intuitive!

• Reasoning: If reduce area, the number of contact points reduces. This causes the pressure to increase at these points, which in turn flattens them more and results in an increase in contact area.

• Overall effect: total “contact area” about the same!

• There are no simple “laws of friction”, as it is affected by several factors, eg:– Surface quality (roughness)– Type of material– Presence of lubricants…

• Lubricants act to separate the two surfaces and allow them to “float” – greatly reducing the friction.





Fall 2004


Spring 2009


PHYS 1800

Lecture 8


Introduction and Review





Fall 2004


Spring 2009

Free Body Diagrams

• Fancy Science: Vector analysis of complex force problems is facilitated by use of a free body diagram.

• Common Sense: A picture is worth a 100 words. (A scale picture is worth an A!)

• Key is to:• Isolate a single body and draw all the forces acting on it. • Add up all the arrows (vectors).• What’s left is the net force.• Net force (and masses) a.• A plus initial conditions motion!





Fall 2004


Spring 2009

Does a sky diver continue to accelerate?

Air resistance R is a force directed upward, that opposes the gravitational force W R increases as the sky diver’s velocity increasesWhen R has increased to the magnitude of W, the net force is zero so the acceleration is zeroThe velocity is then at its maximum value, the terminal velocity





Fall 2004


Spring 2009

Example: Sea Lion splash!

Resolve the weight force into two components – parallel and perpendicular to ramp.

θ

θ

Fk

F||

N

W = m g

mg cosθ

W=mg

NFk

=mg sinθ

Free body diagram

Result: - Down slope force F|| = m g sin θ

F||

θ

- Normal force N = m g cos θ





Fall 2004


Spring 2009

θ

N

mg cosθ

W=mg

Fk

F||=mg sinθ

Net force down slope:

Fnet = F|| - Fk

= m a

but friction, Fk = μk N = μk mg cosθ

Thus: Fnet = m g sinθ – μk m g cosθ = m a

a = g (sinθ - μk cosθ)

a = 9.81 (sin 23º - 0.26 x cos23º)a = 1.5 m/s2 (note: ‘a’ is independent of mass)

For θ = 23º, μk= 0.26, g = 9.81 m/s2, then





Fall 2004


Spring 2009


PHYS 1800

Lecture 8


More Complex Problems in Statics and Dynamics





Fall 2004


Spring 2009

Newton Provided Both the Ideas and the Methods

Hart’s list of most influential people in the history of the world:Newton (2)* Einstein (10)Galileo Galilei (12)*Aristole (13)***Copernicus (19) *Kepler (75) *

*(even though they got the wrong answer on the test)

Simmon’s list of most influential scientists in the history of the world Newton (1)* (and 2 and 6 and 40)Einstein (2)Galileo Galilei (7)*Copernicus (9)Kepler (10)Tyco Brahe (22) Aristole (an honorable mentioned)***





Fall 2004


Spring 2009

How does this trajectory happen?

Key: - resolve motion into its HORIZONTAL and VERTICAL components.

But we know VG increases with time due to gravity acceleration!

At any instant the total velocity is vector sum of VH and VG

Resultant TRAJECTORY STEEPENS with increasing time.

As NO horizontal acceleration the ball movesequal distances horizontally in equal time (assuming NO air resistance).

VH = constant

VG (due to gravity)VTOTAL

Uniform increasein VG

withtime

VH (constant)





Fall 2004


Spring 2009


Next Lab/Demo: Forces

Thursday 1:30-2:45 ESLC 53

Ch 3

Next Class: Wednesday 10:30-11:20

BUS 318 roomRead Ch 4

Mass & Weight Friction Introduction Section 0 Lecture 1 Slide 1 Lecture 8-9 Slide 1 INTRODUCTION TO...

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