Action Reaction Pairs


Newton's third law

If body A exerts a force on body B, then body B will exert an equal and opposite force on body A.

This means that forces always exist in action-reaction pairs. However in every interaction the two paired forces are equal in strength and exist simultaneously. There is no way to identify one force as "action" and the other as "reaction".

Don't think that since these action-reaction pairs are equal and opposite that they cancel out. Equal and opposite forces cancel out only when they are acting on the same body. For example if a book is sitting on a table, then the net force is zero. The weight of the of the book acts down, Fg, and the normal force of the surface of the table on the book, FN, acts up.

These two forces are not an action-reaction pair since they are the result of two separate interactions. Since weight is a downward pull on the book by the earth, the reaction to the weight is an upward pull by the block on the earth. The upward force of the block on the surface.

Examples of action-reaction pairs: