1.2 Physical Quantities and

Their Units

1. A physical quantity is a quantity that can be measured.

2. Physical quantities are measured in S.I. units.

a. S.I. units means International System of Units.

A. Measurement of Physical Quantities.

Base quantities.

1. Prefixes =expressing physical quantities that are either too big or too small.

2. For example, the thickness of a book is measured in centimetres where centi- is the prefix added to metre to express a smaller value.

3. A prefix is added to change the value of the unit.

4. Hence, the value of any physical quantity can be changed to prefix form.

Prefixes used in measurement

Work examples a. Example 1 Write 45 000 g with the kilo prefix. 45000= 45 X 1000(kilo) 45 kg

Symbol and prefixes

1. length-straight line- metre rule (mm/cm) - measuring tape curve line- opisometer and metre rule

thread and metre rule

2. temperature- thermometer

Measuring tools

3. Measure diametera) External diameter-external caliperb) Internal diameter-internal calipers

External diameter=……cmInternal diamerter=…….cmThickness of beaker 1oocm3=

INTER CALIPERS

External Diameter

4. measuring area a) regular object=use mathematical formulae

5cm

2cm

Area = 5cm X 2cm =…………..cm2

b) irregular object

Estimate area use graph paper

Method:

1) Count the number of complete squares.2) An incomplete square is counted if half

or more of the square is covered. 3) Do not count an incomplete square if less than half of the square is covered.

b) irregular object= estimate area use graph paper

1cm

1cm

Area= 27 cm2

Measuring Volume We often need to measure the volume of

liquids. We use a measuring cylinder, burette or

pipette to measure the volume of a liquid.

We usually measure the volume of a liquid in mililitres (ml) and the volume of a solid in cubic centimetres (cm3).

Burette

Pipette

Measuring cylinder

Correct eye position is very important when taking readings from the scale of a measuring tool.

How to read the volume of liquid

To find the volume of a regular object and irregular object.

In this method, the object is dropped into a measuring cylinder. This will cause the water level in the measuring cylinder to rise. The water level rises because the water in the measuring cylinder has been displaced by the object.

Water Displacement Method

y mL

x mL

The volume of water displaced = y-x =50-30 = 20 ml

The volume of the stone = The volume of water displaced.

= (y-x) ml = (y-x) cm3

= 20 ml = 20 cm3

Measuring Volume of the object

30 ml

45 ml

stone

What is the volume of the stone?

= Water displacement method

Volume of the stone = (45 - 30) = 15 cm3

Suitable for volume of uneven object

Measuring Volume object less dance than water

30 cm3

45 cm3Object x

load

55.5 cm3

What is the volume of x?

Volume of object x = (55.5 -45) = 10.5cm3

Volume of stone

WHAT IS WEIGHT? The weight of an object is the pull of the

Earth (force of gravity) on the object. The S.I unit of weight is Newton (N). The weight of any object depends on the

gravitational force. The weight of an object is obtained using a

spring balance or compression spring balance.

spring balance

compression balance

WHAT IS MASS? The mass of an object is the quantity of

matter in the object. The S.I. unit of mass is kilogram (kg). The mass of an object can be obtained

using a triple beam balance or lever balance.

Lever balance

Triple beam balance

Electronic balance

EXERCISE 5: CAN U DIFFERENTIATE BETWEEN MASS AND WEIGHT?

MASS WEIGHT

CAN U DIFFERENTIATE BETWEEN MASS AND WEIGHT?

MASS WEIGHT

It is the amount of matter in an object.

It is the gravitational pull

on an object.

Its value is fixed.Its value varies from place to

place.

Unit: kilogram (kg) Unit: Newton (N)

Measured using beam balance or

lever balance.

Measured using spring balance or weighing balance.

Importance of standard units in

everyday life

The use of standard units make it easier for people from different countries to communicate with each other.

The use of a standard unit means a measurement in that unit has the same value anywhere in the world.

1. Other systems of standard units can be converted to S.I. units and vice versa: a. 1 inch = 2.54 cm b. 1 foot (12 inches) = 30.48 cm c. 1 yard (3 feet) = 0.91 m d. 1 mile = 1.609 km d. 1 mile = 1.609 km e. 1 pound = 0.45 kg f. 1 gallon = 4.5 l