Q. $100 $ balls each of mass $m$ moving with speed $v$ simultaneously strike a wall normally and reflected back with same speed, in time $t\, s$. The total force exerted by the balls on the wall is
Solution:
$P _{ i }= Nm \cup \hat{ i } \,\,\,\, \vec{ P }_{ f }=- Nm v \hat{ i }$
$N$ is Number of balls strikes with wall
$ N =100$
$ \Delta \vec{ P }=\vec{ P }_{ f }-\vec{ P }_{ i }=-2 Nm v \hat{ i }$
$=-200 Nm \hat{\hat{i}} $
$\vec{ F }_{\text {Total }}=\frac{\Delta \vec{ P }}{\Delta t }=-\frac{200 mvt }{ t } $
$ |\vec{ F }|=\frac{200 mv }{ t }$
