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# Floatation and Relative Density

Physics | 11-13 yrs | Interactive, Reading Pod

## What is floatation?

Floatation is the phenomenon of any substance or object resting on the surface of a liquid, without sinking.

### The following are some floatation examples :

• A plastic bottle floats on water.
• A piece of wood floats on water.
• Oil drops floating on water.

## Why does an object float?

An object floats because of the differences in density of the object and the medium which is mainly liquid.
If a substance is denser than a liquid, it will sink. If a substance is less dense than the liquid it is put into, it will float.

## What is density of a substance?

Density is the weight of a substance per unit volume. You can define the density of water by saying how many kilograms a liter of water or kerosene or any other substance weighs.
Another way of calculating density is by looking at the Relative Density of a substance. Which defines how dense a substance is, compared to another substance.

For example :
Mercury is 13.6 times denser than water. So if one liter of water weighs roughly one kilogram, one liter of mercury would weigh 13.6 kilograms.

## What is the formula for Relative Density?

The formula for Relative Density (RD) is :
RD = (Weight of any volume of a substance) / (Weight of an equal amount of water)

Example :
500 ml of citric acid (lemon juice) weighs 800 grams. If 500 ml of water weighs 500 grams. What is the relative density of citric acid?

Solution :
RD = Weight of 500 ml Citric Acid/Weight of 500 ml Water
RD = 800/500
RD = 1.6
The Relative Density of Citric Acid with respect to Water is 1.6

The Relative Densities of some common substances are given below :

 Relative Density of Water 1 Relative Density of Soil 2 Relative Density of Mercury 13.6 Relative Density of Sand, 2.65 Relative Density of Silver 10
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