Question

A block of mass M is hanging over a smooth and light pulley through a light string. T he 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.

Options

• The tension in the string is Mg.

• The tension in the string is F.

• The work one by the tension on the block is 20 J in the above 1 s.

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

Answer 1

 The tension in the string is F.

Tension in the string is equal to F, as tension on both sides of a frictionless and massless pulley is the same.  
i.e., T – Mg = Ma ⇒ T = Mg + Ma
So, the tension in the string cannot be equal to Mg.
The change in kinetic energy of the block is equal to the work done by gravity.
Hence, the work done by gravity is 20 J in 1 s, while the the work done by the tension force is zero.