Newton’s Third Law of Motion

Newton noticed that forces always come in pairs as a result of the interaction between two bodies and that the two forces were always equal in size and opposite in direction.

In his third law Newton states:

If body A exerts a force on body B, then body B exerts a force on body A that is equal in size but opposite in direction.


Newton’s third law can be explained by the example of the space shuttle shown in the animation below:


This is an important point to remember. The two forces in an action-reaction force pair always act on two different objects. Therefore, the two forces can never cancel each other out. The forces acting on the shuttle are the weight and thrust (although their will also be a drag force caused by friction in the atmosphere). The thrust is the reaction force from the exhaust gases. The action force i.e. the exhaust gases from the rear of the shuttle are not acting on the rocket rather the rocket pushes on the exhaust gases.