Question

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 $F$. If kinetic energy of the block increases by $20J$ in $1s$. Then:

• A. Tension in the string is $Mg$.
• B. Tension in the string is $F$.
• C. Work done by the tension on the block is $20J$ in $1sec$.
• D. Work done by the force of gravity is $-20J$ in $1sec$.

Answer 1

The correct option is B Tension in the string is $F$.

Tension in the string would be equal to the force exerted on the other end of the string. Thus $T=F$.

Net work done on the body is equal to the gain in kinetic energy of the block.

Thus increase in kinetic energy in $1s=$Work done by tension on the block in $1s+$Work done by gravity on the block in $1s$.

$⟹20J=W_T+W_G$

Thus $W_T\ne 20J$

$W_G\ne 20J$