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Transcript
Chapter 6: Newton’s Third Law of Motion Action and Reaction THINK 1.) What is Newton’s first law of motion? 2.) What is Newton’s second law of motion? 3.) A 30 kg refrigerator is pulled down a loading ramp with a force of 45 N. Find the acceleration of the refrigerator. 4.) What is a force? Newton’s Third Law of Motion Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. One force is called the action force, the other force is called the reaction force. It doesn’t matter which force we call the action force and which we call the reaction force – they are both equal in magnitude and always opposite in direction. Newton’s law can be stated in other words: For every action, there is an equal and opposite reaction. A ball is dropped off of a building and falls to the ground. Earth exerts a force on the ball (the force of gravity). In turn, the ball exerts a force on earth. The force that the ball exerts on earth is the same as the force that earth exerts on the ball. The ball accelerates toward earth as a result of the action force. Does this mean that earth accelerates toward the ball, as a result of the reaction force? Yes! Why do we not notice the acceleration of earth towards the ball? Recall Newton’s Second Law of Motion: a = F / m The force acting on the ball and acting on earth is the same, but the masses are much different. Because earth has such an enormous mass, the acceleration is incredibly small, too small for us to even witness! Action and Reaction Forces DO NOT CANCEL Even though action and reaction forces are equal in magnitude and opposite in direction, they do not cancel each other out. Forces that are equal in magnitude and opposite in direction cancel each other out only when they are acting on the same object. A child pulls a wagon down the street Force that wagon exerts on child Force that child exerts on wagon Friction force Friction force Think 1.) What is Newton’s third law of motion? 2.) Identify the action and reaction forces in each situation: (a) A tennis ball is hit by a tennis racket (b) An apple falls to the ground (c) A car crashes into a guardrail 3.) Suppose a friend who hears about Newton’s Third Law of Motion says that you can’t move a football by kicking it because the reaction force by the kicked ball would be equal and opposite to you kicking force. The net force would be zero, so no matter how hard you kick, the ball won’t move. What do you say to your friend? Draw a diagram for each of the following situations, just like we did with the child and the wagon. Label all the horizontal forces acting on each object. 1.) A person pushing a crate along the sidewalk 2.) A person walking his dog 3.) A child pulls another child on a sled In each situation, what is the action force? What is the reaction force?