Newton's Third Law and Momentum

Core concept

When a gun is fired, the forward force propelling the bullet has an equal and opposite recoil force pushing the gun backward — this is Newton's Third Law in action. In any closed system with no external forces, total momentum is conserved before and after a collision, a principle called the Law of Conservation of Momentum, which explains phenomena like why a swimmer pushes water backward to move themselves forward, and is fundamental to understanding collisions in vehicle safety design.