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.Then

• A. tension in the string is $40N$
• B. displacement of the block in the given interval of time is $2m$
• C. work done by gravity is $-20J$
• D. work done by tension is $80J$

Answer 1

The correct option is B displacement of the block in the given interval of time is $2m$

OPTION B is the correct answer

A force of $40N$ is applied on the string.

So,

$F-mg=ma$

$40-20=ma$

$a=\frac{20}{m}=\frac{20}{2}=10{ms}^{-2}$

Also, $\Delta k\cdot E=\frac{1}{2}m{v}^2=40$

$v^2=\frac{40\times 2}{m}=\frac{40\times 2}{2}$

$v^2=40$

Using eqn of motion

$2as=v^2-u^2$

$s=\frac{v^2-u^2}{2a}=\frac{40-0}{2\times 10}=2m$

$Displacement=2m$

$Note-$40 N is the external force not the Tension