Four identical balls. A, B, C and D are placed in line on a frictionless horizontal surface. A and D are moved with the same speed 'u' towards the middle as collisions, find the velocities.
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Solution
A and D collide especially with B and C, respectively, and come to rest but B and C starts moving with velocity u towards each other as shown. B and C collide especially and exchange their velocities to move in the opposite direction. Now, B and C collide elastically with A and D respectively, and come to rest but A and D start moving with velocity u away from each other as shown. Therefore, final velocities. VA=u(←),VB=0,VC=0 and VD=u(→).