Action-reaction pairs include a swimmer pushing off a wall, helicopters creating lift by pushing air down, and an octopus propelling itself forward by ejecting water from its body.Two equal and opposite forces do not cancel because they act on different systems.Newton’s third law of motion represents a basic symmetry in nature, with an experienced force equal in magnitude and opposite in direction to an exerted force.Physics Question Pack Passage 17 Question 101 Thus, they do not cancel each other.Įlectrostatics, medicine, and metal spheres In other words, the two forces are distinct forces that do not act on the same body. Second, these forces are acting on different bodies or systems: A’s force acts on B and B’s force acts on A. First, the forces exerted (the action and reaction) are always equal in magnitude but opposite in direction. There are two important features of Newton’s third law. The vertical forces w and BF cancel because there is no vertical acceleration. Thus, the free-body diagram shows only F wall on feet, w (the gravitational force), and BF, which is the buoyant force of the water supporting the swimmer’s weight. The line around the swimmer indicates the system of interest. This opposition occurs because, in accordance with Newton’s third law, the wall exerts a force F wall on feet on the swimmer that is equal in magnitude but in the direction opposite to the one she exerts on it. When the swimmer exerts a force on the wall, she accelerates in the opposite direction in other words, the net external force on her is in the direction opposite of F feet on wall. This reaction force, which pushes a body forward in response to a backward force, is called thrust. She pushes the wall in the direction behind her, therefore the wall will exert a force on her that is in the direction in front of her and propel her forward. This is because the wall exerts the same force on her that she forces on it. The more force she exerts on the wall, the harder she pushes off. Newton’s third law has practical uses in analyzing the origin of forces and understanding which forces are external to a system.įor example, rake a swimmer who uses her feet to push off the wall in order to gain speed. We sometimes refer to this law loosely as “action-reaction,” where the force exerted is the action and the force experienced as a consequence is the reaction.
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