Similarly, a car accelerates because the ground pushes forward on the car's wheels in reaction to the car's wheels pushing backward on the ground. The floor exerts a reaction force in the forward direction on the teacher that causes him to accelerate forward. As a teacher paces in front of a whiteboard, he exerts a force backward on the floor.

Other examples of Newton’s third law are easy to find. This opposition is the result of Newton’s third law of motion, which dictates that the wall exerts a force F wall on feet F wall on feet on the swimmer that is equal in magnitude but that acts in the direction opposite to the force that the swimmer exerts on the wall. This means that the net external force on her is in the direction opposite to F feet on wall. Note that the swimmer pushes in the direction opposite to the direction in which she wants to move.įigure 4.8 When the swimmer exerts a force F feet on wall F feet on wall on the wall, she accelerates in the direction opposite to that of her push. Therefore, F feet on wall F feet on wall does not directly affect the motion of the system and does not cancel F wall on feet. Because the swimmer is our system (or object of interest) and not the wall, we do not need to consider the force F feet on wall F feet on wall because it originates from the swimmer rather than acting on the swimmer. Because acceleration is in the same direction as the net external force, the swimmer moves in the direction of F wall on feet. If we choose the swimmer to be the system of interest, as in the figure, then F wall on feet F wall on feet is an external force on the swimmer and affects her motion. In this case, there are two different systems that we could choose to investigate: the swimmer or the wall. 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.

Law of physics