Consider two point masses, `m_1` and `m_2`, moving along a straight line in the same direction with speeds `u_1`, and `u_2`. Let them undergo one-dimensional collision and retrieve each other with velocities `v_1` and `v_2`.how that (`u_1`- `u_2`) = - (`v_1` -`v_2`) i.e, after collision, their relative velocities are equal.