Q. 105.0( 1 Vote )

A block of mass M is hanging over a smooth and light pulley through a light string. The other end of the string is pulled by a constant force F. The kinetic energy of the block increases by 20 J in 1 s.
A. The tension in the string is Mg.

B. The tension in the string is F.

C. The work done by the tension on the block is 20 J in the above 1 s.

D. The work done by the force of gravity is -20 J in the above 1 s.

Answer :

Option A is not true because if the tension in the string is Mg the block cannot be pulled dup. It can only happen when the tension in the string is F, which is the case of option B, hence it is true.


Since the kinetic energy of the block increases by 20J, it means work done by the resultant force on the block is 20J. But neither the tension nor the gravitational force is the resultant force on the block, hence options C and D are not correct

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