question_answer 13)

(i) A child stands at the centre of turntable with his two arms out stretched. The turntable is set rotating with an angular speed of40 rpm How much is the angular speed of the child if he folds his hands back and thereby reduces his moment of inertia to 2/3 times the initial value ? Assume that the turntable rotates without friction. (ii) Show that the child's new kinetic energy of rotation more than the initial kinetic energy of rotation. How do you account for this increase in kinetic energy ?  

Answer:

Here By the principle of conservation of angular momentum,         or            or           . (ii) Initial kinetic energy of rotation   New kinetic energy of rotation   . Thus the child's new kinetic energy of rotation is 2.5 times its initial kinetic energy of rotation. This increase in kinetic energy is due to the internal energy of the child which he uses in folding his hands back from the out stretched position.