Two equal masses m₁ and m₂ moving along the same straight line with velocities +3 m s⁻¹ and -5 m s⁻¹ respectively collide elastically. Their velocities after the collision will be respectively

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(4) -5 m s⁻¹ and +3 m s⁻¹. Given: equal masses at +3 m s⁻¹ and -5 m s⁻¹, colliding elastically head on. Idea: for equal masses an elastic head-on collision simply exchanges the two velocities. Check it against the general result: v₁=(m₁-m₂)/(m₁+m₂)u₁+(2m₂)/(m₁+m₂)u₂. With m₁=m₂ the first term vanishes and the second becomes u₂, so v₁=u₂=-5 m s⁻¹. Likewise v₂=u₁=+3 m s⁻¹. Momentum before and after is -2m, and the kinetic energy is unchanged, as an elastic collision requires.

Two equal masses m₁ and m₂ moving along the same straight line with velocities +3 m s⁻¹ and -5 m s⁻¹ respectively collide elastically. Their velocities after the collision will be respectively

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