The first collision takes place at time t1 and the second collision takes place at time t2. Find t2−t1 :
Let the velocities of the cart and the bead just after the first collision are Vc and Vb respectively. Momentum is conserved just before and just after the collision:
MV0=MVc+mVb......(1)(−V0)e=Vb−Vc(Coefficientofrestitution)⇒Vo=Vc−Vb(sincee=1)......(2)⇒Vc=(M−m)(M+m)V0
⇒Vb=2M(M+m)V0
with respect to the cart the ball has to move a distance of L for the second collision. time between two collisions is
t=LVbc=L2M(M+m)V0−(M−m)(M+m)V0=LV0