Rotational Motion Question 83

Question: A mass m moves in a circle on a smooth horizontal plane with velocity $ v _0 $ at a radius $ R _0 $ . The mass is attached to a string which passes through a smooth hole in the plane as shown. The tension in the string is increased gradually and finally m moves in a circle of radius $ \frac{R _0}{2} $ . The final value of the kinetic energy is

Options:

A) $ mv_0^{2} $

B) $ \frac{1}{4}mv_0^{2} $

C) $ 2mv_0^{2} $

D) $ \frac{1}{2}mv_0^{2} $

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Answer:

Correct Answer: C

Solution:

  • Conserving angular momentum

    $ L _{i}=L _{t} $

    $ \Rightarrow $ $ mv _0R _0=mv’( \frac{R _0}{2} ) $

    $ \Rightarrow $ $ v’=2v _0 $

    So, final kinetic energy of the particle is

    $ K _{f}=\frac{1}{2}mv{{’}^{2}}=\frac{1}{2}m{{(2v _0)}^{2}} $

    $ =4\frac{1}{2}mv_0^{2}=2mv_0^{2} $



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