State the law of conservation of angular momentum and explain with a suitable example.
Statement: The angular momentum of a body remains constant, if resultant external torque acting on the body is zero.
a. A ballet dancer makes use of the law of conservation of angular momentum to vary her angular speed.
b. Torque acting on her body is zero. By law of conservation of angular momentum, L = constant
Iω = constant ⇒ ω ∝1/I
c. When she suddenly folds her arms and brings the stretched leg close to the body, her angular velocity increases on account of decrease in moment of inertia. This helps in rotating safely on her legs.