Unit 1: Phenomenon The Physics of Skydiving Lesson 3c Newton s Third Law of Motion California Standard Addressed PH1. Newton s laws predict the motion of most objects. As a basis for understanding this concept: 1. d. Students know that when one object exerts a force on a second object, the second object always exerts a force of equal magnitude and in the opposite direction (Newton s third law). Student Performance Outcomes Identify that forces occur in pairs. Given the magnitude and direction of a force in an interaction, predict the magnitude and direction of the opposing force in the pair. State and explain Newton s Third Law of motion. DO NOT WRITE ON THIS HANDOUT Engage 1. When you exert a force on a balloon, what does the balloon exert on you? 2. Is it possible to exert a force without a counter force being exerted? If so give an example. 3. Two cars have a head on collision. How does the size of the force exerted by the red car compare to the size of the force exerted by the silver car? Explain your thinking. 4. A tennis ball strikes a tennis racket. How does the size of the force exerted by the ball compare to the size of the force exerted by the racket? Explain your thinking. (www.rit.edu/~andpph/exhibit-8.html; taken by Andrew Davidhazy) 5. A bug hits a windshield of a car. How does the size of the force exerted by the bug compare to the size of the force exerted by the car? Explain your thinking. A person exerts a net force of 8N to a pair of blocks. 6. What is the size (magnitude) of the force exerted by block A on the hand? 7. What is the magnitude of the force exerted by block B on block A? Share your answers with your lab group. Come to consensus with your answers and share them with the class. 1
Explore Your task is to make a series of predictions about the magnitude of forces exerted by interacting objects. Once you have made your predictions, test your ideas. 8. Same size people push hard on each other. 9. One person is bigger, people push hard on each other. 10. One person is bigger, smaller person pushes, the bigger person resists. 11. Bigger person pushes, the smaller person resists while braced against a wall. 2
13. Same size people are pulling on spring scales. 14. One person is bigger, both are pulling. 15. Same size people are pulling on spring scales with a rubber band between. 16. Same size people are pulling on spring scales with a rubber band and a spring between them Explain 17. Summarize your findings from a previous set of experiments. 18. For each situation, how does the magnitude of the force exerted by one person compare with the magnitude of the force exerted by the other person? 19. What effect(s) did the rubber band and/or stiff spring have on these forces? 20. When a force is exerted, there is always a counter force of equal and opposite direction. 3
Let us introduce a definition. Two objects that exert pushes or pulls on each other are said to interact with each other. An interaction always involves two objects that are doing the interaction. The objects can be people, springs scales, or any of a number of different things. Consider the following statement that summarizes Newton s Third Law of Motion: When two objects interact, the magnitude of the force that one object exerts on the other is equal to the magnitude of the force that the second object exerts on the first, and in the opposite direction. Really important: The forces are the same on each object but the effects are different when the masses of the two objects are different. The same force on the car windshield and the fly will have a dramatically different effect on the fly. 21. Does the evidence you gathered in today s experiments support this statement? 22. How will the force exerted on the cannon ball compare to the force exerted on the cart? 23. What is the result of the force exerted on the cart? 24. What exerts a force on the rocket that pushes it forward? Elaborate Your teacher will demonstrate three set-ups where two force sensors interact and display the forces acting on each force sensor on the Promethean board. I. Moving tug of war Connect two force sensors together. Gently pull against one another and monitor the force measured by each sensor. II. Pulling against a stationary point Secure one force sensor to an anchored stand. Connect the two force sensors together. Gently pull and monitor the force measured by each sensor. III. Forces in moving systems Place some weights on one of the sensors. Connect the two force sensors together. Gently drag one sensor and monitor the force measured by each sensor. 4
Data Sketch the appearance of the force vs. time graph as measured by each sensor. I. II. III. 25. How do the magnitudes of the peak forces measured by each sensor compare in I? 26. How do the magnitudes of the peak forces measured by each sensor compare in II? 27. How do the magnitudes of the peak forces measured by each sensor compare in III? Evaluate 28. The students drawn above are holding bathroom scales. They push against one another as the picture shows. If the scale on the left reads 40.N, the scale on the right will read... a) Greater than 40. newtons. b) Less than 40. newtons. c) 40. newtons. d) It depends on whether or not the people move. 29. The large truck drawn above collides with the small car on the right. The size of the push exerted by the truck on the car will be... a) smaller than the size of the push exerted by the car on the truck. b) greater than the size of the push exerted by the car on the truck. c) the same size as the push exerted by the car on the truck. d) zero. 30. The spring is released and both carts are allowed to move freely. M=2kg. Which cart will experience a greater average force? a) cart m c) neither cart will experience a force b) cart 2m d) the average force exerted on each cart will be the same Use the graphic and information from question 30 above. 31. 32. Which cart will experience a greater average acceleration? a) cart m c) neither cart will experience an acceleration b) cart 2m d) the carts will experience the same acceleration Your weight is the result of the gravitational force of the earth on your body. Which of the following describes the reaction force? a) The gravitational force you exert on the earth b) The normal force exerted on you by the ground c) The friction force exerted on you by the ground. d) none of the above 5
33. 34. If you step off a ledge, you accelerate noticeably toward the earth because of the gravitational interaction between you and the earth. Does the earth accelerate towards you as well? a) Yes, and it is easily measurable. b) Yes, but the inertia of the earth makes it difficult to detect. c) No. The size of the force exerted by the car on the bug is... a) smaller than the size of the force exerted by the bug on the car. b) greater than the size of the force exerted by the bug on the car. c) the same size as the push exerted by the bug on the car. d) zero. A bug smashes into the window of a speeding car. 6
One of the students in this class makes the following statement: 35. "If a big football player runs into a little cameraman on the side of the field, the football player has to exert the biggest force. It's because he weighs so much more." This statement is supported by a) Newton s first law c) Newton s third law b) Newton s second law d) None of Newton s laws 36. John pulls his spring scale to the left and Mary pulls her spring scale to the right. Mary s scale reads 5 newtons of force. The scales move steadily to the right with a speed of 10 cm/second. John pulls with a force... a) greater than 5 newtons b) less than 5 newtons c) 5 newtons d) 0 newtons CART A collides into CART B as shown above. Force measuring probes are attached to each cart and measure the force exerted in the collision. The data from the probes is recorded below. 37. Average Force Measured by Probe on Cart A Average Force Measured by Probe on Cart B 1.27 N 1.27 N Which is the best interpretation of these results? a) The probes are broken. b) This data supports the Newton s 3rd law of motion. c) This data does not support Newton s 3rd law of motion. d) This data supports Newton s 1st law of motion. How much force does the water exert on the swimmer? 38. a) 50N b) greater than 50N c) less than 50N d) 0 N the water moves out of the way. A swimmer exerts 50N of force on the water. 7
39. 40. 41. A pair of blocks are push along a frictionless surface as shown below. What is the size and direction of the force exerted by block A on block B (just the force on block B)? What is the direction and magnitude of the force exerted by block B on Block A? What is the size and direction of the force exerted by the hand (total force on both blocks)? a) 6 newtons to the right b) 15 newtons to the right c) 9 newtons to the right d) 6 newtons to the left e) 15 newtons to the left a) 6 newtons to the right b) 15 newtons to the right c) 9 newtons to the right d) 6 newtons to the left e) 15 newtons to the left a) 6 newtons to the right b) 15 newtons to the right c) 9 newtons to the right d) 6 newtons to the left e) 15 newtons to the left 42. State and explain Newton s Third Law of Motion. Give an example to illustrate why the effect is not necessarily the same on each object. 8