STUDY GUIDE UNIT 10-Newton s Third Law

Transcription

1 Name ANSWERS STUDY GUIDE UNIT 10-Newton s Third Law Date Agenda HW Tues, Jan 5 Wed., Jan 6 Review Video Read Section Fill in Reading Notes (p. 2) Worksheet - Action-Reaction Pairs (p. 3) Go over worksheet - Action-Reaction Pairs Video Clips- (p. 6-7) Read Section Fill in Reading Notes (p. 9-10) Video Notes - Part 1 (p. 4-5) Video Notes - Part 2 (p. 11) Thurs., Jan 7 Problem Set 1 (p ) Problem Set 2 (p ) Study for Quest Fri., Jan 8 Go over Problem Sets 1 and 2 Quest 1

2 CP Physics Reading Notes Section Newton realized that a force is not a thing in itself but part of a mutual action, an INTERACTION, between one thing and another 2. Describe the interaction between a hammer and a nail: THE INTERACTION THAT DRIVES THE NAIL IS THE WSAME AS THE ONE THAT STOPS THE HAMMER 3. State Newton s Third Law: WHENEVER ONE OBJECT EXERTS A FORCE ON A SECONDS OBJECT, THE SECOND OBJECT EXERTS AN EQUAL AND OPPOSITE FORCE ON THE FIRST OBJECT 4. One force is called the ACTION force, and the other is called the REACTION force 5. To every ACTION there is always an equal and opposing REACTION 6. Forces are equal in STRENGTH and opposite in DIRECTION 7. In figure 6.5, describe the action and reaction for each force pair. ACTION: TIRE PUSHES ROAD ACTION: ROCKET PUSHES GAS ACTION: EARTH PULLS BALL REACTION; ROAD PUSHES TIRE REACTION GAS PUSHED ROCKET REACTION: BALL PULLS EARTH 2

3 Video Notes-Part 1 3

4 Newton's Third Law of Motion - "Action and Reaction" Directions: Take notes on this organizer as we discuss these topics in class. I. Force: II. Newton's Third Law III. Identifying Action and Reaction Force Pairs 4

5 IV. Equal and Opposite Forces are NOT ALWAYS an Action Reaction Pair Example Cup Resting on a Table Newton's Third Law of Motion Collisions 5

6 In a collision between a minivan and a train locomotive: Which experiences the largest (in magnitude) force? Explain. Which experiences the largest effect (i.e. largest acceleration) and why? Newton's Third Law of Motion How Rockets Work 6

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8 CP Physics Reading Notes Section Figure 6.6: Earth is pulled by the boulder with JUST AS MUCH FORCE as the boulder is pulled down by the Earth 2. Newton s Second Law states that ACCELERATION is not only proportional to the NET FORCE, but it is also inversely proportional to the MASS 3. Figure 6.7: The force exerted against the recoiling cannon is just as great as the force that drives the cannonball along the barrel. Why, then does the cannonball undergo more acceleration than the cannon? THE CANNONBALL OF THE CANNON BALL IS MUCH LESS THAN THE MASS OF THE CANNON 4. Since action and reaction forces are equal and opposite, why don t they cancel out to zero? ACTION AND REACTION DO NOT CANCEL OUT BECAUSE EACH ACTS ON A DIFFERENT OBJECT 8

9 5. Look at the Horse-Cart Problem a. What is the net force that acts on the cart in Figure 6.13? THE NET FORCE ON THE CART IS THE DIFFERENCE BETWEEN THE PULL OF THE HORSE AND THE FRICTION FORCE ON THE CART WHEELS b. What is the net force that acts on the horse in Figure 6.13? THE NET FORCE ON THE HORSE IS THE DIFFERENCE BETWEEN THE PUSH OF THE HORSE AND THE GROUND ON EACH OTHER AND THE PULL OF THE HORSE AND THE CART ON EACH OTHER 9

10 c. What is the net force that acts on the ground in Figure 6.13? THE NET FORCE ON THE GROUND IS THE DIFFERENCE BETWEEN THE PUSH OF THE HORSE AND THE GROUND ON EACH OTHER AND THE FRICTION FORCE BETWEEN THE CART WHEELS AND THE GROUND. d. Explain, using forces, why the horse-cart system accelerates: THE HORSE CARE SYSTEM ACCELERATES DUE TO THE DIFFERENCE BETWEEN THE PUSH OF THE HORSE AND THE GROUND ON EACH OTHER AND THE FRICTION FORCE BETWEEN THE CART WHEELS AND THE GROUND. 10

11 Video Notes-Part 2 Newton's Third Law of Motion - "Action and Reaction" I. Action-Reaction Forces Acting on Objects of Different Masses Newton's 2nd Law: Newton's 3rd Law: Acceleration of each object Bullet: = Rifle: = II. Why Don't Action and Reaction Forces Cancel? BUT Action Reaction Pair Forces on the Ball ACTION: RESULT: REACTION: 11

12 Problem Set 1 Newton's 3rd Law Questions Directions: Answer the following questions in complete sentences 1. In the interaction between a hammer and the nail it hits, is a force exerted on the nail? On the hammer? How many forces occur in this interaction? THE HAMMER EXERTS A FORCE ON THE NAIL AND THE NAIL EXERTS A FORCE ON THE HAMMER. THEY ARE EQUAL AND OPPOSITE. THERE ARE TWO FORCES. 2. When a hammer exerts a force on a nail, how does the amount of force compare with that of the nail on the hammer? THE FORCES ARE THE SAME. BOTH THE HAMMER AND THE NAIL ARE PART OF ONE INTERATION. 3. When you walk along a floor, what pushes you along? Include a diagram in your answer. YOU PUSH ON THE FLOOR AND THE FLOOR PUSHES YOU FORWARD 4. When you jump up, the world really does recoil downward. Why can t this motion of the world be noticed? THE ACCELERATION IT TOO SMALL BECAUSE OF THE WORLD S LARGE MASS 5. When a rifle is fired, how does the size of the force of the rifle on the bullet compare with the force of the bullet on the rifle? How does the acceleration of the rifle compare with that of the bullet? Explain your answer. THE FORCE THE RIFLE APPLIES TO THE BULLET IS EQUAL AND OPPOSITE TO THE FORCE THE BULLET APPLIES TO THE RIFLE. THE ACCELERATION IS SMALL FOR THE RIFLE BECAUSE OF ITS LARGER MASS, AND THE ACCLEREATION OF THE BULLET IS LARGE BECAUSE OF ITS SMALLER MASS 12

13 6. Your weight is the result of the gravitational force of the earth on your body. What is the corresponding reaction force? THE REACTION FORCE IS THE FORCE EXERTED BY YOU ON THE EARTH 7. If you walk on a log that is floating in the water, the log moves backward. Why? YOU APPLY OF FORCE TO THE LOG AND THE LOG EXERTS AN EQUAL AND OPPOSITE FORCE ON YOU. THE FORCE OF THE LOG ON YOU ACCELERATED YOU FORWARD AND THE FORCE OF YOU ON THE LOG ACCELERATED IT BACKWARDS 8. If a bicycle and a massive truck have a head-on collision, upon which vehicle is the impact force greater? Which vehicle undergoes the greater change in its motion? Defend your answers. THE IMPACT FORCE IS THE SAME ON BOTH. BECAUSE OF THE BICYCLE S SMALLER MASS, THE CHANGE IN MOTION (ACCELERATION) IS GREATER FOR THE BICYCLE. 9. A speeding bus makes contact with a bug that splatters onto the windshield. Because of the sudden force, the unfortunate bug undergoes a sudden deceleration. Is the corresponding force that the bug exerts against the windshield greater, less, or the same? Is the resulting deceleration of the bus greater than, less than, or the same as that of bug? THE FORCE OF THE BUG ON THE BUS IS EQUAL AND OPPOSITE TO THE FORCE OF THE BUS ON THE BUG. THE BUG UNDERGOES A MUCH GREATER DECELERATION THAN THE BUS BECAUSE IT HAS A MUCH SMALLER MASS 10. Some people used to think that a rocket could not travel to the moon because it would have no air to push against once it left the earth s atmosphere. We now know that idea was mistaken. What force propels a rocket when it is in a vacuum? THE FORCE IS THAT WHICH THE EXHAUST EXERTS. IT IS THE REACTION TO THE FORCE THE ROCKET EXERTS ON THE EXHAUST GASES. 13

14 Problem Set 2 Newton s Third Law Think and Explain 1. Why is it easier to walk on a carpeted floor than on a smooth, polished floor? YOU CAN EXERT A GREATER HORIZONTAL FORCE ON A CARPET THAN ON A POLISHED FLOOR BECAUSE OF THE GREATER FRICTION. THIS IN TURN PROVIDES A GREATER REACTION FORCE TO PROVIDE TRACTION FOR WALKING 2. 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 toward you as well? Explain. YES, THE EARTH ACCELERATED TOWARD YOU AS WELL, BUT THE ACCELERATION PRODUCED BY THESE EQUAL FORCES ARE QUITE UNEQUAL BECAUSE OF THE GREAT DIFFERENCE IN MASSES BEING ACCELERATED 3. Suppose you re weighing yourself while standing next to the bathroom sink. Using the idea of action and reaction, explain why the scale reading will be less when you push down on the top of the sink. WHEN YOU PUSH DOWN ON THE SINK, THE SINK PUSHES YOU UPWARD. THIS TENDS TO LIFT YOU OFF THE SCALE AND DECREASE THE READING ON THE SCALE Why will the scale reading be more if you pull up on the bottom of the sink IF YOU PULL UPWARD ON THE SINK, IT WILL PUSH DOWNWARD ON YOU AND INCREASE THE READING 14

15 6. When a high jumper leaves the ground, what is the source of the upward force that accelerates her? THE GROUND PUSHES UPWARD ON THE JUMPER, WHICH IS THE FORCE THAT PROVIDES AN UPWARD ACCELERATION. What force acts after her feet are no longer in contact with the ground? IN THE AIR, ONLY THE FORCE OF GRAVITY ACTS ON THE JUMPER. UPWARD ACCELERATION CEASES AND THE ACCELERATION IS DOWNWARD (g) 7. Since the force that acts on a bullet when a gun is fired is equal and opposite to the force that acts on the gun, does this imply a zero net force and therefore the impossibility of an accelerating bullet? Explain. IN THIS CASE, THERE ARE TWO SETS OF INTERACTIONS. THE EXPANDING GASES IN THE GUN AT ON BOTH THE BULLET AND THE GUN. ACTION AND REACTION DO NOT CANCEL BECAUSE EACH ACTS ON A DIFFERENT OBJECT ONE ON THE BULLET AND ON THE GUN 15

16 8. The strong man can withstand the tension force exerted by the two horses pulling in opposite directions. How would the tension compare if only one horse pulled and the left rope were tied to tree? THE TENSION WOULD BE THE SAME How would the tension compare if the two horses pulled in the same direction, with the left rope tied to the tree? SINCE THERE IS TWICE THE FORCE, THERE WOULD BE TWICE THE TENSION ON THE ROPE. 16