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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