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STATE THE LAW OF CONSERVATION OF ANGULAR MOMENTEM AND EXPLAIN IT WITH AN EXAMPLE

Conservation of Angular Momentum.
The total angular momentum of a system of particle is constant when the net external torque acting on the system is zero.
That is L = I? = constant.
This is called the ‘law of conservation of angular momentum’. If I decreases, ? increases and vice versa.
If the external torque is zero ( =0), then
         or    L = constant
or I? = constant
If the moment of inertia changes from I1 to I2 due to change of the distribution of mass of the body, then angular velocity of the body changes from ?1 to ?2. Such that
        I₁ ?₁  = I₂ ?₂
Or        
Or               
where T is time period
Some examples of conservation of angular momentum are as follows:
a.    When a planet revolving around the Sun in an elliptical orbit comes near the Sun, its angular velocity increases. This is because as the planet comes near the Sun, its distance decreases and hence, its moment of inertia decreases and hence according to law of conservation of angular momentum, its angular velocity increases.
b.    When a diver jumps into water from a height, he does not keep his body straight but pulls in his arms and legs towards the centre of his body. On doing so, the moment of inertia of his body decreases and his angular velocity increases according to law of conservation of angular momentum.