Motion and forces Study Guide

When two objects interact, Newton's third law states that the force object A exerts on object B is equal in magnitude and opposite in direction to the force B exerts on A. These forces are of the same type and always act on different objects. In practical terms, during collisions, these equal and opposite forces are responsible for the exchange of momentum between the objects. The change in momentum (impulse) each object experiences is equal in size but opposite in direction. Provided there are no significant external forces (an isolated system), the total momentum before the collision is the same as after, showing the conservation of momentum arises directly from Newton’s third law.

Momentum is a vector quantity calculated by multiplying an object's mass by its velocity. In collisions, the principle of conservation of momentum states that the total momentum in a closed system remains constant if no external forces act on it. This is true for both elastic collisions, where objects bounce apart and both momentum and kinetic energy are conserved, and inelastic collisions, where objects may stick together and only momentum is conserved. For example, the collision of billiard balls is nearly elastic, while vehicles crumpling together in an accident is an inelastic collision—that is, momentum is conserved in both cases, but kinetic energy behaves differently.