Two Particle of equal masses `4 M` are initially at rest. A particle of mass `M` moving at speed `u` collide elastically with one of the larger balls. How many collision occure?

Answer

2

Explanation

Consider first collision `b//w` `M and 4M` on the right, the velocities after collision are
`V_(M)= -(3)/(5)u, V_(4M)=(2)/(5)u`
Particle of mass `M` will move to the left and collide with `4M`. The velocities after collision are
`V_(M)=((3)/(5))^(2)u, V_(4M)=(2)/(5)xx(3)/(5)u`
So in all there are two collision.