Relative Density Class 9 Physics

Hello students, in this article we will study the important condition of substance which decides whether the substance is going to float or sink in a liquid, termed as relative density.

Before proceeding with relative density directly, we shall memorise the meaning of density. We have learned in previous classes that, ‘ratio of mass per unit volume of substance is called as density’. Density of substance decides the strength of substance. Substances like metal have higher density than non metals; hence metals are tougher than that of non metals.

Suppose we want to study the substance with respect to paraffin. Then we shall define the term relative density as, ‘ratio of density of substance per unit density of paraffin.’ In short, relative density is ratio of density of substance per unit density of reference substance.

Generally relative density is studied by keeping water as reference substance, and then it will be defined as, ‘the ratio of density of substance to that of density of water’.

Since relative density is ratio of similar quantities, it is unitless.

Using Archimedes principle it is always convenient to discuss the nature of substance. You may have noticed that liquids like kerosene or petrol when added in beaker containing water, two layers of liquid i.e. kerosene and water can be easily seems in beaker. This indicates that the density of kerosene is less than that of water so it easily floats on water.

Archimedes principle states that, “if object is immersed in liquid completely or partially, the buoyant force exerted by liquid on an object is equal to weight of liquid which is displaced by the object”.

By Archimedes principle magnitude of buoyant force is given as,

∴ W = ρVg …………………………..(2)

As per Archimedes principle, this weight is nothing but the buoyant force hence,

∴ Buoyant force,F = ρVg …………..(3)

Due to relative density the weight of object in any liquid then written as,

Weight of object in liquid (apparent weight) = Actual weight- buoyant force.

If the value of relative density with respect to water is…

  1. Greater than 1: substance will sink into water.
  2. Less than 1: substance will float on water.
  3. Equal to 1: substance will float inside the water.

Applications of relative density:

  1. Relative density decides whether the substance is going to float or sink in liquid.
  2. By knowing relative density one can find the unknown density of substance.

Consider the following numerical to illustrate the relative density.

Ex:1) The density of honey is 1400 kg/m3 added to water of density 1000 kg/m3, what is the relative density of water with respect to honey?

Ans: density of honey =1400 kg/m3

density of water = 1000 kg/m3

Ex:1) Relative density of a substance with respect to water is 1.2, what is the density of substance? Hence state whether the substance will sink or float in water.

Ans: Relative density =1.2

density of water = 1000 kg/m3

Relative density is greater than 1, hence substance will sink in to water.

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