Whenever one body exerts a force on another, the latter simultaneously exerts an equal and opposite force on the first. In vector form, if F 1 , 2 is the force of body 1 on body 2 and F 2 , 1 that of body 2 on body 1, then F 1 , 2 = − − F 2 , 1 . This law is sometimes referred to as the action-reaction law, with F 1 , 2 called the action and − − F 2 , 1 the reaction.