Question

A block of mass $2kg$ is hanging over a smooth and light pulley through a light string. The other end of the string is pulled by a constant force $F=40N$. The kinetic energy of the particle increases $40J$ in a given interval of time. Work done by tension is

• A. $100J$
• B. $120J$
• C. $80J$
• D. $40J$

Answer 1

Answer: (C)

$\Rightarrow a=10m/s^2$

$40-20=2a$

Let $ui=0$

$v=at=10t$

$s=\frac{1}{2}\times 10v^2=5v^2$

$\Rightarrow \frac{1}{2}\times 2\times \left(10t^2\right)=40$

$\Rightarrow 100t^2=40$

$\Rightarrow W_T=40\times 5v^2=200v^2=80J$

Hence,

option $\left(C\right)$ is correct answer.