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(a) A child stands at the centre of a tu...

(a) A child stands at the centre of a turntable with his two arms outstretched. The turntable is set rotating with an angular speed of 40 rev/min. How much is the angular speed of the child if he folds his hands back and thereby reduces his moment of inertia to 2/5 times the initial value ? Assume that the turntable rotates without friction.
(b) Show that the child’s new kinetic energy of rotation is more than the initial kinetic energy of rotation. How do you account for this increase in kinetic energy?

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(a) 100 rev/min (use angular momentum conservation).
(b) The new kinetic energy is 2.5 times the initial kinetic energy of rotation. The child uses his internal energy to increase his rotational kinetic energy.
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NCERT GUJARATI-SYSTEMS OF PARTICLES AND ROTATIONAL MOTION-EXERCISES
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  2. A solid cylinder of mass 20 kg rotates about its axis with angular spe...

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  4. A rope of negligible mass is wound round a hollow cylinder of mass 3 k...

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  11. A solid cylinder rolls up an inclined plane of angle of inclination 30...

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  19. Explain why friction is necessary to make the disc in Fig. 7.41 roll i...

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