A particle of mass m strikes another particle of same mass at rest. Find the angle between the velocities of the particles after the collision, if the collision is elastic.

Since the collision is elastic and both particles have the same mass, according to the law of conservation of momentum and due to the symmetrical nature of the problem, the first mass will transfer its momentum to the second mass. The velocity of the first particle will become zero, and the second particle will move with the initial velocity of the first particle. Hence, the angle between the velocities of the particles after the collision will be Zero.