Newton s third law relates action and reaction forces.

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1 Chapter 11, Section 3 Key Concept: Forces act in pairs. BEFORE, you learned A force is a push or a pull Increasing the force on an object increases the acceleration The acceleration of an object depends on its mass and the force applied to it NOW, you will learn How Newton s third law relates action/reaction pairs of forces How Newton s laws work together THINK ABOUT How do jellyfish move? Jellyfish do not have much control over their movements. They drift with the current in the ocean. However, jellyfish do have some control over their up-and-down motion. By squeezing water out of its umbrella-like body, the jellyfish shown here applies a force in one direction to move in the opposite direction. If the water is forced downward, the jellyfish moves upward. How can a person or an object move in one direction by exerting a force in the opposite direction? Newton s third law relates action and reaction forces. Newton made an important observation that explains the motion of the jellyfish. He noticed that forces always act in pairs. Newton s third law states that every time one object exerts a force on another object, the second object exerts a force that is equal in size and opposite in direction back on the first object. As the jellyfish contracts its body, it applies a downward force on the water. The water applies an equal force back on the jellyfish. It is this equal and opposite force on the jellyfish that pushes it up. This is similar to what happens when a blown-up balloon is released. The balloon pushes air out the end, and the air pushes back on the balloon and moves it forward. check your reading What moves the jellyfish through the water? Action and Reaction Pairs The force that is exerted on an object and the force that the object

2 exerts back are known together as an action/reaction force pair. One force in the pair is called the action force, and the other is called the reaction force. For instance, if the jellyfish pushing on the water is the action force, the water pushing back on the jellyfish is the reaction force. Likewise, if the balloon pushing the air backward is the action force, the air pushing the balloon forward is the reaction force. You can see many examples of action and reaction forces in the world around you. Here are three: You may have watched the liftoffs of the space shuttle on television. When the booster rockets carrying the space shuttle take off, their engines push fuel exhaust downward. The exhaust pushes back on the rockets, sending them upward. When you bang your toe into the leg of a table, the same amount of force that you exert on the table is exerted back on your toe. Action and reaction forces do not always result in motion. For example, if you press down on a table, the table resists the push with the same amount of force, even though nothing moves. check your reading Identify the action/reaction forces in each example described above. Action and Reaction Forces Versus Balanced Forces Because action and reaction forces are equal and opposite, they may be confused with balanced forces. Keep in mind that balanced forces act on a single object, while action and reaction forces act on different objects. Balanced Forces If you and a friend pull on opposite sides of a backpack with the same amount of force, the backpack doesn t move, because the forces acting on it are balanced. In this case, both forces are exerted on one object the backpack. Action and Reaction As you drag a heavy backpack across a floor, you can feel the backpack pulling on you with an equal amount of force. The action force and the reaction force are acting on two different things one is acting on the backpack, and the other is acting on you. The illustration below summarizes Newton s third law. The girl exerts an action force on the boy by pushing him. Even though the boy is not trying to push the girl, an equal and opposite reaction force acts upon the girl, causing her to move as well.

3 When one object exerts a force on another object, the second object exerts an equal and opposite force on the first object. Newton s Third Law 1 One Skater Pushes 2 Both Skaters Move How does the direction of the force on the girl relate to her motion? boy in motion. Newton s Three Laws of Motion Newton s three laws describe and predict motion. Newton s three laws can explain the motion of almost any object, including the motion of animals. The illustrations on page 60 show how all three of Newton s laws can be used to describe how kangaroos move. The three laws are not independent of one another; they are used together to explain the motion of objects. You can use the laws of motion to explain how other animals move as well. For example, Newton s laws explain why a squid moves forward while squirting water out behind it. These laws also explain that a bird is exerting force when it speeds up to fly away or when it changes its direction in the air.

4 You can also use Newton s laws to make predictions about motion. If you know the force acting upon an object, then you can predict how that object s motion will change. For example, if you want to send a spacecraft to Mars, you must be able to predict exactly where Mars will be by the time the spacecraft reaches it. You must also be able to control the force on your spacecraft so that it will arrive at the right place at the right time. Knowing how Newton s three laws work together can also help you win a canoe race. In order to start the canoe moving, you need to apply a force to overcome its inertia. Newton s second law might affect your choice of canoes, because a less massive canoe is easier to accelerate than a more massive one. You can also predict the best position for your paddle in the water. If you want to move straight ahead, you push backward on the paddle so that the canoe moves forward. Together, Newton s laws can help you explain and predict how the canoe, or any object, will move. Questions for Chapter 11, Section 3 KEY CONCEPTS 1. Identify the action/reaction force pair involved when you catch a ball. 2. Explain the difference between balanced forces and action/reaction forces. 3. How do Newton s laws of motion apply to the motion of an animal, such as a cat that is running? CRITICAL THINKING 4. Apply A man pushes on a wall with a force of 50 N. What are the size and the direction of the force that the wall exerts on the man? 5. Evaluate Jim will not help push a heavy box. He says, My force will produce an opposite force and cancel my effort. Evaluate Jim s statement as either True or False, and explain your reasoning.

5 CHALLENGE 6. Calculate Suppose you are holding a basketball while standing still on a skateboard. You and the skateboard have a mass of 50 kg. You throw the basketball with a force of 10 N. What is your acceleration before and after you throw the ball

SECTION 1 (PP ):

SECTION 1 (PP ): FORCES CHANGE MOTION. Georgia Standards: S8P3b Demonstrate the effect of balanced and unbalanced forces on an object in terms of gravity, inertia, and friction; S8CS6a Write clear, step-by-step instructions