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Give different examples to illustrate the phenomena of conservation of angular momentum.


Conservation of momentum

Law of conservation of angular momentum states that in the absence of an external torque, the angular momentum of a system remains constant.

Acrobats (an acrobat is a circus performer walking on a tight fastened rope), divers, ballet dancers (do you know ballet dance?), ice skaters and others to perform breath-taking acts use this law. Following is a number of feats used in performances.

Have you ever seen (or experienced) some youngsters making the fun of wearing some special shoes with tires in it, and perform on the roads like ice-skaters. They are continuously stretching their arms out or closing it to their bodies in order to have the balance.
Sometimes they also hold sticks in their hands. Do you know why?

(1) Diver

A diver leaves the springboard and jumps into the pond. The springboard provides the angular speed about a horizontal axis through the center of gravity. The angular speed is small as his arms and legs are fully extended. When he pulls his arms and legs inside, his moment arm ‘I’ decreases. As angular momentum is ‘Iω’, it should also decrease. However, it does not happen because of the law of conservation of momentum must hold. As a consequence as the moment arm ‘I’ decreases, the angular velocity ω increases thereby the angular momentum remains constant. Thus, he becomes able to make few more somersaults.

(2) Circus acrobat

An acrobat standing on turning table decreases his angular velocity by stretching his arms out and increasing his angular velocity by drawing in his arms close to the chest. It is because by stretching out or drawing in his arms his angular velocity ω decreases or increases, respectively and hence, his angular momentum remains constant.

(3) Revolving stone

When we revolve a stone fastened to one end of the rope by exerting force on it. After sometimes, we stop exerting force on the rope and allow it to wind around the finger. As the rope winds, the length of the rope decreases and the angular velocity increases at the same time so that the product mrω (= angular momentum) remains constant.


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