You might think that two forces of equal magnitude but that act in opposite directions would cancel, but they do not because they act on different systems. The wall has thus exerted on the swimmer a force of equal magnitude but in the direction opposite that of her push. She pushes against the pool wall with her feet and accelerates in the direction opposite to her push. Consider a swimmer pushing off from the side of a pool, as illustrated in Figure 4.8. We can see Newton’s third law at work by looking at how people move about. Recall that identifying external forces is important when setting up a problem, because the external forces must be added together to find the net force. Newton’s third law is useful for figuring out which forces are external to a system. We sometimes refer to these force pairs as action-reaction pairs, where the force exerted is the action, and the force experienced in return is the reaction (although which is which depends on your point of view). Newton’s third law of motion tells us that forces always occur in pairs, and one object cannot exert a force on another without experiencing the same strength force in return. Newton’s third law of motion states that whenever a first object exerts a force on a second object, the first object experiences a force equal in magnitude but opposite in direction to the force that it exerts. Everyday experiences, such as stubbing a toe or throwing a ball, are all perfect examples of Newton’s third law in action. This is exactly what happens whenever one object exerts a force on another-each object experiences a force that is the same strength as the force acting on the other object but that acts in the opposite direction. Although the first thought that crosses your mind is probably “ouch, that hurt” rather than “this is a great example of Newton’s third law,” both statements are true. If you have ever stubbed your toe, you have noticed that although your toe initiates the impact, the surface that you stub it on exerts a force back on your toe. Ask students which forces are internal and which are external in each scenario. Explain how forces can be classified as internal or external to the system of interest. Introduce the concepts of systems and systems of interest. Start a discussion about action and reaction by giving examples.

Three laws in physics science