Class 9 Science


Buoyancy is the upward force exerted by fluids over the surface area of contact of an object which is immersed in fluids. Buoyancy is also known as upward thrust.

Why does an object sink or float over water?

When an object is immersed in water, it exerts pressure over water due to its weight. At the same time water also exerts upward thrust over the object. If the force exerted by the object is greater than the upward thrust or buoyancy by water, the object sinks in water otherwise it floats over water.

Buoyancy in everyday life:

Swimming in water: Anyone can be able to swim in water because of upward thrust exerted by water.

Flying of bird or aero plane: Since air is a fluid, thus it also exerts upward thrust over the object. Therefore, because of upward thrust of air a bird or aero plane can fly in air.

Factors which affect buoyancy

Volume of the object: Buoyancy or upward thrust exerted by a fluid increases with the volume of the object immersed in it.

Density of the fluid: The buoyant force or upward thrust increases with increase in density of the fluid. Denser liquid exert more upward thrust.

This is the cause that it is easier to swim in sea water rather than fresh water. Sea water is saline. Salts dissolved in sea water increase the density and hence it exerts more upward thrust than fresh water.


Mass per unit volume of an object is called density or mass density. Density is denoted by Greek letter rho (ρ).

`=>rho = m/V`

Where, m is mass of object and V is the volume of that object.

SI Unit of density

The SI unit of mass is kg and SI unit of volume is cubic meter.

Therefore, by substituting the unit of mass and volume in the expression of density we get

`rho = (kg)/m^3`

Therefore, SI unit of density `(rho)` is `kg\ m^(-3)`

Relative density

For convenience the density of water is expressed in comparison of water. Thus, when density of a substance is expressed in comparison with water, it is called relative density.

∴ `text{Relative density}` `=(text{Density of substance})/(text{Density of water})`

Relative density has no unit, because it is the ratio of similar quantity.

Practical application of density

Water is taken as reference material because it is easily available at most of the places and it is easy to carry out the measurement related to density in field visit.

When the relative density of a substance is less than 1, it will float in water otherwise it will sink in water.

The relative density of ice is 0.91, thus, it floats in water.

If the relative density of an object is less than 1, compare to the liquid in which it is immersed, the object will sink otherwise object will float.

Archimedes’ Principle

Archimedes’ Principle states that when an object is immersed fully or partially in a liquid, it experiences an upward force which is equal to the weight of liquid displaced by the object.

Application of Archimedes’ Principle

When the weight of displaced liquid by an object is greater than the weight of object, the object will float in liquid and when the weight of liquid will be smaller than that of the weight of the object, the object will sink in liquid.

In submarines: In submarines, there is a tank which can be filled or emptied as per requirement. It is called buoyancy tank.

When submarines have to go inside the water, the buoyancy tank is filled with water, so that, the weight of the submarines would become more than the weight of water displaced by it. And when submarines have to come over the surface of water, the buoyancy tank is emptied and water is replaced by air. In this condition, the weight of water displaced by submarines will become more than the weight of submarines and submarine comes over the surface of water.

Air balloons: To raise the air balloon in air; the air inside the balloon is heated. Air expands because of heat and becomes lighter. Thus, the balloon gets and upward thrust from the surrounding air and rises up.

Ships: A ship is much heavier than water, yet it floats on water. This happens because of unique shape of the ship. Because of its shape, the volume of the ship is larger compared to its weight. Due to this, water displaced by the ship provides a proper upward thrust to the ship and the ship floats on water.