# A ball is moving with velocity 3m/s towards a heavy wall moving towards the ball with speed 1m/s. Assuming collision to be elastic, find the velocity of ball immediately after collision

$(a)\;-5 \;m/s \quad (b)\;4\;m/s\quad (c)\;5\;m/s \quad (d)\;4\; m/s$

Since collision is elastic $e=1=\large\frac{v_2-v_1}{u_1-u_2}$
Speed of wall will not change after collision
v be velocity of ball after collision .
Relative speed of seperation = relative speed of approach
$v_2-v_1=u_1-u_2$
$u_1=3 m/s$
$u_2=-1 m/s$
$-1-v=3-(-1)$
$v=-5m/s$
Hence a is the correct answer.

edited Feb 10, 2014 by meena.p