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# A ball is moving with velocity v strikes a wall moving towards the ball with velociy u. An elastic impact lasts for t seconds. Find the mean elastic force acting on the ball

$(a)\;\frac{2m(v-u)}{t} \quad (b)\; \frac{2m(v+u)}{t} \quad (c)\;\frac{2m(v+u)}{t(v-u)}\quad(d)\;\frac{m(v+u)}{t}$

Relative speed of ball w.r.t wall is $v+u$
Therefore $F=\large\frac{Change \;in\;momentum}{time}$
$\qquad=\large\frac{m\bigg[(u+v)-\{-(v+u)\}\bigg]}{t}$
$\qquad=\large\frac{2m(v+u)}{t}$
Hence b is the correct answer.

edited Jan 26, 2014 by meena.p