2masses undergo elastic collision as shown in figure. A plane mirror is moving with a speed 3v as shown. After collision what is the relative velocity of mass 1 after collision with respect to its image?

In an elastic collision between equal masses, their velocities get exchanged. Hence the mass 1 moves with velocitytowards the mirror after collision.

But the velocity of mass with respect to mirror istowards the mirror. So the image will also move towards the mirror(on the other side) with velocity. Hence the velocity of mass 1 with respect to its image is.