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Newton’s Third Law of Motion concerns equal and opposite forces. It states that, “If A exerts a force on B, then B exerts an equal but opposite force on A”.

Forces always occur in ‘Newton pairs’ and are made up from an action force and an equal reaction force in the opposite direction.

This video shows how this works in two real-life situations involving skateboarders and rockets.

Although momentum is conserved in this collision, calculation of kinetic energy before and after the collision shows kinetic energy is not conserved. This is an inelastic collision.

If kinetic energy before is the same as after, the collision is elastic. Interactions between molecules are perfectly elastic collisions. In most other cases (eg snooker balls), collisions are not perfectly elastic – some kinetic energy is lost but this must be tested by calculation.