Two billiard balls of mass 0 . 05   kg each moving in opposite directions with 10   ms - 1 collide…

After collision, the balls change their directions of motion without changing the magnitudes of their velocity.

Change in momentum of any one ball

$\left|\Delta \overrightarrow{p}\right|=2\times 0.05\times 10$

$\Rightarrow \left|\Delta \overrightarrow{p}\right|=1 \mathrm{kg} \mathrm{m} {\mathrm{s}}^{-1}$

 The force exerted on the ball is $\left|{\overrightarrow{F}}_{\mathrm{av} }\right|=\frac{\left|\Delta \overrightarrow{p}\right|}{\Delta t}=\frac{1}{0.005}$

$F_{\mathrm{av} }=200 \mathrm{N}$