Question

A body of mass $10kg$ is moving on a horizontal surface by applying a force of $10N$ in forward direction. If body moves with constant velocity, the work done by force of friction for a displacement of $2m$ is :

• A. $-20J$
• B. $10J$
• C. $20J$
• D. $-5J$

Answer 1

The correct option is A $-20J$

As the body is moving with constant velocity, thus the net external force acting on the body must be equal to zero.

Hence the friction force must be equal to the external force applied on the body.

$\therefore$ Frictional force $f=F=10$ $N$ (acting in opposite direction to that of velocity)

Distance moved by the body $S=2$ $m$

$\therefore$ Work done by friction $W_f=FScos\theta$ where $\theta ={180}^o$

$W_f=FScos{180}^o=10\times 2\times (-1)=-20J$