In any collision, momentum is conserved; but in an elastic collision, kinetic energy is also conserved. Consider particles A and B with masses m A, m B, and velocities v A1, v B1 before collision, v A2, v B2 after collision. The conservation of momentum before and after the collision is expressed by: m A v A 1 + m B v B 1 = m A v A 2 + m B v B 2 .