Page 92 - Physics

Physics for Secondary Schools

The mass of the 20 ml of liquid can be Example 4.6
obtained by subtracting the mass of an
3
empty beaker from the combined mass of An object has a density of 7 g/cm .
the beaker and liquid. Calculate its relative density, (R.D).

Solution
The density of liquid can be calculated Density of object = 7 g/cm 3
( m - m ) g
using the formula 1 0 ⋅ Since the R.D = Density of substance
20 cm 3 Density of water
beaker is used to measure the mass of the 7 g /cm 3
liquid, its mass has to be known. Relative density = 1 g /cm 3 = 7

Relative density of a substance Therefore, relative density of the
object is 7.
It is common practice to express the

density of a substance such as solid, Example 4.7
liquid and gas (air) in relation to the
density of water. The ratio between A piece of copper metal of volume
3
density of substance to the density of 5.1 cm has a mass of 41.6 g. Calculate
water is called relative density, R.D. the relative density of copper.
Mathematically, it can be expressed as; Solution
Mass of the piece of copper = 41.6 g
Density of substance
R.D = Volume of the piece of copper = 5.1 cm 3
Density of water
mass
Mass of substance density of copper =
= volume
Mass of equal volume of water
41.6 g
The relative density has no unit, since = 5.1 cm 3
it is expressed as the ratio of the same
quantity. = 8.16 g/cm .
3

Relative density is also known as the Relative density = Density of substance
specifi c gravity (S.G). It indicates Density of water
how many times a substance is denser 8.16 g / cm 3
compared to water. A substance is denser RD = 1 g /cm 3

than water if its relative density is larger
than 1 and the substance is less dense than = 8.16.
water if its relative density is less than 1.
Therefore, relative density of copper is 8.16.

86
Student’s Book Form One

Physics Form 1 Final.indd 86 16/10/2024 20:56