1.1 - 1.3Introduction to Physics & the Quantity

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1.1 Introduction to Physic

Understanding Physics

The study of the laws that determine the structure of the universe with reference to the matter and energy of

which it consists.

The study was divided into separated fields; heat , the properties of matter, light, sound ,wave, electricity,

magnetism, mechanics, nuclear physics etc.

In physics , there is the need to make careful observations, precise and accurate measurements.

Understanding natural phenomena and observing everyday objects such as a table, a mirror etc and discuss

how they are related to physics concepts has always been a central aim of physics.

The roots of all science are firmly based in experiment. Of course , mastering scientific skills applying

scientific knowledge must be the important thing to learn physics.

Physical Quantities

Physics is based on measurement. We discover physics by learning how to measure the quantities that are

involved in physics and we call its as physical quantities.

The meaning of Physical Quantities

Physical quantities are quantities that can be measured.

Examples of physical quantities are length, mass, time, weight, pressure, current and force.

A physical quantity is a property ascribed to phenomena, objects, or subtances that be quantified.

Example are:

(i) Frequency of oscillation frequency is the quantity and oscillation is the phenomenon

(ii) Length of a wooden block Length is the quantity and the wooden block is an object

(iii) Density of water density is the quantity and the water is the substance

There are two types of the physical quantities,

Base quantities

Derived quantities

Base Quantities

The physical quantities which are used as the basis for the measurement and cant be derived from

other physical quantities.

There are five base quantities as shown in the following table:

Base Symbol Unit Unit Measured
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quantity

name

Abbreviation

by

Length l meter m Ruler

Mass m Kilogram kgTriple Balance

beam

Time t Second s Stopwatch

Temperature T Kelvin K Thermometer

Current I Ampere A Ammeter

Derived Quantities

The physical quantities which were derived from base quantities by multiplication operation or

division operation or both

There are three examples for derived quantities as shown in the following table.

Derived

quantity

Symbol In term of the

base quantities

Derived unit

Area A m x m m2

Velocity v m s-1

Density kg m-3

1.2 Unit of Measurement

Introduction

Physics is an experimental science. Theories are useful only if their predictions agree with the results of

experiments. So measurement plays an important part in physics.Together with other scientists , physicists

have agreed on a single system of units for the measurement of physical quantities .

A physical quantity is clearly defined with a numerical value and a unit. A physical quantity can be

measured using a standard size called the unit.

Units identify the quantity that has been measured.

The standard size used must

(i) be easily reproduced

(ii) not have its magnitude changed

(iii) be internationally accepted

Although various systems of units have been used over the years, scientists have generally agreed to use the

International System of Units (S.I. Unit).

Units for the base quantities are known as base units, i.e. metre , kilogram, second, kelvin and ampere.

The definition of base units.
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The metre is the length of the path traveled by light in vacuum during a time interval of 1299,792,458 of a

second

The kilogram is the mass of a prototype cylinder of platinum-iridium alloy that is kept

at the International Bereau of Weights and Measures.

The second is 9192631770 periods of a specified radiation from Cesium-133 atoms

The kelvin is 1/ 273.16 of the thermodynamic temperature of the triple point of water

The ampere is the current carried by two long parallel wires placed one meter apart,

when the attractive force per unit length between two wires is 2 x 10 -7Nm-1.

Prefixes

Prefix is a scientific notation word before the base units and have certain value uses powers of 10.

Factor Name Symbol Factor Name Symbol

1024 yotta Y 10-1 deci d

1021 zetta Z 10-2 centi c

1018 exa E 10-3 milli m

1015 peta P 10-6 micro

1012 tera T 10-9 nano n

109 giga G 10-12 pico p

106 mega M 10-15 femto f

10

3

kilo k 10

-18

atto a102 hecto h 10-21 zepto z

101 deka da 10-24 yocto y

100 One -

Standard Form

The radius of earth 6400000000 mm and the diameter a metal wire is 0.00000045 km. What is the best way

to write these numbers ?

The best way is try to write the numbers in shorthand form.

The shorthand form of writing numbers is called standard form.

In general, the standard form number is

a x 10n

where

and n are positive integers or negative integers.
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so 6 400 000 000 mm becomes 6.4 x 109mm

1.3 Scalar and vector

> A scalar quantity is a physical quantity which has only magnitude. For example, mass, speed(laju), density, pressure, .

> A vector quantity is a physical quantity which has magnitude and direction. For example, force,momentum, velocity (halaju), acceleration .

Graphical representation of vectors

A vector can be represented by a straight arrow,

The length of the arrow represents the magnitude of the vector.

The vector points in the direction of the arrow.

Basic principle of vectors

Two vectors P and Q are equal if:

(a) Magnitude of P = magnitude of Q

(b) Direction of P = direction of Q

When a vector P is multiplied by a scalar k, the product is k P and the direction remains the sameas P.

The vector -P has same magnitude with P but comes in the opposite direction.

Sum of vectors

Method 1: Parallelogram of vectors

It two vectors and are represented in magnitude and direction by the adjacent sides OAand OB of a parallelogram OABC, then OC represents their resultant(paduan).

Method 2: Triangle of vectors

Use a suitable scale to draw the first vector.
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From the end of first vector, draw a line to represent the second vector.

Complete the triangle. The line from the beginning of the first vector to the end of the secondvector represents the sum in magnitude and direction.

Example 4

A kite flies in still air is 4.0 ms-1. Find the magnitude and direction of the resultant velocity of thekite when the air flows across perpendicularly(serenjang) is 2.5 ms-1. If the distance of the kite is30 m,

what is the time taken for the kite to fly? Calculate the height of the kite from the ground.

Vector 1- direction (yes)

2- magnitude (c2=a2+b2)

c= 4.7m s-1

Principles of vectors

Relative velocity

Let us look at two cases: VA = 10 ms-1 VB = 3 ms

-1.

Case one

The velocity ofA relative to B = (VA VB)

= (10- 3) ms

= 7 ms -1 (in forward direction).
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Case two

The velocity ofB relative to A = (VB VA)

= (3 10) ms

= -7 ms -1 (in backwards direction).

We observe that(VB VA) and (VA VB) are same magnitude but different direction.

Resolving(leraian) vector

A vector R can be considered as the two vectors. R refers to the resultant vectors. There are twomutually perpendicular component Rx and Ry

Example 5

The figure shows 3 forces F1, F2 and F3 acting on a point O. Calculate the resultant force and thedirection of resultant.

F1 F2 F3magnitude 3N 5N 4N

Direction

degree 0 150 240

Resolving X-axis

F1x=+3N

X-axis

F2x=-4.3N
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Y-axis

F1y= 0

Y-axis

F2y =2.5N

1.1 - 1.3Introduction to Physics & the Quantity

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