Q.

A mass m moves in a circle on a smooth horizontal plane with velocity v0 at a radius R0. 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 it moves in a circle of radius R0/2. The final value of the kinetic energy is
 

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a

12mv02

b

mv02

c

14mv02

d

2mv02

answer is D.

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Detailed Solution

According to law of conservation of angular momentum Li = Lf

mv0R0=mvR02v'=2v0

Final K.E of the particle is 

Kf=12mv2=12m2v02=2mv02

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A mass m moves in a circle on a smooth horizontal plane with velocity v0 at a radius R0. 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 it moves in a circle of radius R0/2. The final value of the kinetic energy is