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© 2007 Pearson Prentice Hall This work is protected by United States copyright laws and is provided solely for the use of instructors in teaching their courses and assessing student learning. Dissemination or sale of any part of this work (including on the World Wide Web) will destroy the integrity of the work and is not permitted. The work and materials from it should never be made available to students except by instructors using the accompanying text in their classes. All recipients of this work are expected to abide by these restrictions and to honor the intended pedagogical purposes and the needs of other instructors who rely on these materials. Clicker Question ConcepTests PRS-Enabled Chapter 9 Physics, 3 rd Edition James S. Walker

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ConcepTest 9.1Rolling in the Rain ConcepTest 9.1 Rolling in the Rain A) speeds up B) maintains constant speed C) slows down D) stops immediately An open cart rolls along a frictionless track while it is raining. As it rolls, what happens to the speed of the cart as the rain collects in it? (assume that the rain falls vertically into the box)

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A system of particles is known to have a total kinetic energy of zero. What can you say about the total momentum of the system? A) momentum of the system is positive B) momentum of the system is positive C) momentum of the system is zero D) you cannot say anything about the momentum of the system ConcepTest 9.2Momentum and KE I ConcepTest 9.2 Momentum and KE I

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A system of particles is known to have a total momentum of zero. Does it necessarily follow that the total kinetic energy of the system is also zero? A) yes B) no ConcepTest 9.3Momentum and KE II ConcepTest 9.3 Momentum and KE II

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Two objects are known to have the same momentum. Do these two objects necessarily have the same kinetic energy? A) yes B) no ConcepTest 9.4Momentum and KE III ConcepTest 9.4 Momentum and KE III

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A net force of 200 N acts on a 100-kg boulder, and a force of the same magnitude acts on a 130-g pebble. How does the rate of change of the boulder’s momentum compare to the rate of change of the pebble’s momentum? A) greater than B) less than C) equal to ConcepTest 9.5Momentum and Force ConcepTest 9.5 Momentum and Force

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A net force of 200 N acts on a 100-kg boulder, and a force of the same magnitude acts on a 130-g pebble. How does the rate of change of the boulder’s momentum compare to the rate of change of the pebble’s momentum? A) greater than B) less than C) equal to The rate of change of momentum is, in fact, the force. Remember that F = p/ t. Since the force exerted on the boulder and the pebble is the same, then the rate of change of momentum is the same. ConcepTest 9.5Momentum and Force ConcepTest 9.5 Momentum and Force

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A) greater than B) less than C) equal to ConcepTest 9.6Velocity and Force ConcepTest 9.6 Velocity and Force A net force of 200 N acts on a 100-kg boulder, and a force of the same magnitude acts on a 130-g pebble. How does the rate of change of the boulder’s velocity compare to the rate of change of the pebble’s velocity?

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A) greater than B) less than C) equal to The rate of change of velocity is the acceleration. Remember that a = v/ t. The acceleration is related to the force by Newton’s 2 nd Law (F = ma), so the acceleration of the boulder is less than that of the pebble (for the same applied force) because the boulder is much more massive. ConcepTest 9.6Velocity and Force ConcepTest 9.6 Velocity and Force A net force of 200 N acts on a 100-kg boulder, and a force of the same magnitude acts on a 130-g pebble. How does the rate of change of the boulder’s velocity compare to the rate of change of the pebble’s velocity?

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ConcepTest 9.7Collision Course ConcepTest 9.7 Collision Course A) the car B) the truck C) they both have the same momentum change D) can’t tell without knowing the final velocities A small car and a large truck collide head-on and stick together. Which one has the larger momentum change?

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ConcepTest 9.8Two Boxes I ConcepTest 9.8 Two Boxes I FF light heavy A) the heavier one B) the lighter one C) both the same Two boxes, one heavier than the other, are initially at rest on a horizontal frictionless surface. The same constant force F acts on each one for exactly 1 second. Which box has more momentum after the force acts ?

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In the previous question, which box has the larger velocity after the force acts? A) the heavier one B) the lighter one C) both the same ConcepTest 9.9Two Boxes II ConcepTest 9.9 Two Boxes II

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ConcepTest 9.10Watch Out! ConcepTest 9.10 Watch Out! You drive around a curve in a narrow one-way street at 30 mph when you see an identical car heading straight toward you at 30 mph. You have two options: hit the car head-on or swerve into a massive concrete wall (also head-on). What should you do? A) hit the other car B) hit the wall C) makes no difference D) call your physics prof!! E) get insurance!

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A small beanbag and a bouncy rubber ball are dropped from the same height above the floor. They both have the same mass. Which one will impart the greater impulse to the floor when it hits? A) the beanbag B) the rubber ball C) both the same ConcepTest 9.11Impulse ConcepTest 9.11 Impulse

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A small beanbag and a bouncy rubber ball are dropped from the same height above the floor. They both have the same mass. Which one will impart the greater impulse to the floor when it hits? A) the beanbag B) the rubber ball C) both the same change in momentum for the ball is greater, because of the rebound Both objects reach the same speed at the floor. However, while the beanbag comes to rest on the floor, the ball bounces back up with nearly the same speed as it hit. Thus, the change in momentum for the ball is greater, because of the rebound. The impulse delivered by the ball is twice that of the beanbag. For the beanbag: p = p f – p i = 0 – (–mv ) = mv For the rubber ball: p = p f – p i = mv – (–mv ) = 2mv ConcepTest 9.11Impulse ConcepTest 9.11 Impulse Follow-up: Which one imparts the larger force to the floor?

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A person stands under an umbrella during a rainstorm. Later the rain turns to hail, although the number of “drops” hitting the umbrella per time and their speed remains the same. Which case requires more force to hold the umbrella? A) when it is hailing B) when it is raining C) same in both cases ConcepTest 9.12Singing in the Rain ConcepTest 9.12 Singing in the Rain

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ConcepTest 9.13Going Bowling I ConcepTest 9.13 Going Bowling I p p A) the bowling ball B) same time for both C) the ping-pong ball D) impossible to say A bowling ball and a ping-pong ball are rolling toward you with the same momentum. If you exert the same force to stop each one, which takes a longer time to bring to rest?

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ConcepTest 9.14Going Bowling II ConcepTest 9.14 Going Bowling II p p stopping distance A bowling ball and a ping-pong ball are rolling toward you with the same momentum. If you exert the same force to stop each one, for which is the stopping distance greater? A) the bowling ball B) same distance for both C) the ping-pong ball D) impossible to say

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ConcepTest 9.14Going Bowling II ConcepTest 9.14 Going Bowling II p p W = KE less massgreater speed(why?)greater KE(why again?) W = Fd same less massive ballbigger Use the work-energy theorem: W = KE. The ball with less mass has the greater speed (why?), and thus the greater KE (why again?). In order to remove that KE, work must be done, where W = Fd. Since the force is the same in both cases, the distance needed to stop the less massive ball must be bigger. stopping distance A bowling ball and a ping-pong ball are rolling toward you with the same momentum. If you exert the same force to stop each one, for which is the stopping distance greater? A) the bowling ball B) same distance for both C) the ping-pong ball D) impossible to say

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ConcepTest 9.15Elastic Collisions I ConcepTest 9.15 Elastic Collisions I v 2 v 1 at rest A) situation 1 B) situation 2 C) both the same Consider two elastic collisions: 1) a golf ball with speed v hits a stationary bowling ball head-on. 2) a bowling ball with speed v hits a stationary golf ball head-on. In which case does the golf ball have the greater speed after the collision?

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ConcepTest 9.16Inelastic Collisions I ConcepTest 9.16 Inelastic Collisions I vfvf vivi M M M M A box slides with initial velocity 10 m/s on a frictionless surface and collides inelastically with an identical box. The boxes stick together after the collision. What is the final velocity? A) 10 m/s B) 20 m/s C) 0 m/s D) 15 m/s E) 5 m/s

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ConcepTest 9.17Inelastic Collisions II ConcepTest 9.17 Inelastic Collisions II vivi vfvf On a frictionless surface, a sliding box collides and sticks to a second identical box which is initially at rest. What is the final KE of the system in terms of the initial KE? A) KE f = KE i B) KE f = KE i / 4 C) KE f = KE i / 2 D) KE f = KE i / 2 E) KE f = 2 KE i

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ConcepTest 9.17Inelastic Collisions II ConcepTest 9.17 Inelastic Collisions II vivi vfvf On a frictionless surface, a sliding box collides and sticks to a second identical box which is initially at rest. What is the final KE of the system in terms of the initial KE? A) KE f = KE i B) KE f = KE i / 4 C) KE f = KE i / 2 D) KE f = KE i / 2 E) KE f = 2 KE i Momentum: mv i + 0 = (2m)v f v f = 1/2 v i So we see that: v f = 1/2 v i Now, look at kinetic energy: First, KE i = 1/2 mv i 2 So: KE f = 1/2 m f v f 2 = 1/2 (2m) (1/2 v i ) 2 = 1/2 (2m) (1/2 v i ) 2 = 1/2 ( 1/2 mv i 2 ) = 1/2 ( 1/2 mv i 2 ) = 1/2 KE i = 1/2 KE i

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ConcepTest 9.18Nuclear Fission I ConcepTest 9.18 Nuclear Fission I A uranium nucleus (at rest) undergoes fission and splits into two fragments, one heavy and the other light. Which fragment has the greater momentum? A) the heavy one B) the light one C) both have the same momentum D) impossible to say 1 2

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ConcepTest 9.19Nuclear Fission II ConcepTest 9.19 Nuclear Fission II A) the heavy one B) the light one C) both have the same speed D) impossible to say 1 2 A uranium nucleus (at rest) undergoes fission and splits into two fragments, one heavy and the other light. Which fragment has the greater speed?

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Amy (150 lbs) and Gwen (50 lbs) are standing on slippery ice and push off each other. If Amy slides at 6 m/s, what speed does Gwen have? (A) 2 m/s (B) 6 m/s (C) 9 m/s (D) 12 m/s (E) 18 m/s 150 lbs 50 lbs ConcepTest 9.20Recoil Speed I ConcepTest 9.20 Recoil Speed I

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ConcepTest 9.21Recoil Speed II ConcepTest 9.21 Recoil Speed II A) 0 m/s B) 0.5 m/s to the right C) 1 m/s to the right D) 20 m/s to the right E) 50 m/s to the right A cannon sits on a stationary railroad flatcar with a total mass of 1000 kg. When a 10-kg cannon ball is fired to the left at a speed of 50 m/s, what is the recoil speed of the flatcar?

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When a bullet is fired from a gun, the bullet and the gun have equal and opposite momenta. If this is true, then why is the bullet deadly? (whereas it is safe to hold the gun while it is fired) A) it is much sharper than the gun B) it is smaller and can penetrate your body C) it has more kinetic energy than the gun D) it goes a longer distance and gains speed E) it has more momentum than the gun ConcepTest 9.22Gun Control ConcepTest 9.22 Gun Control

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ConcepTest 9.23Crash Cars I ConcepTest 9.23 Crash Cars I A) I B) II C) I and II D) II and III E) all three If all three collisions below are totally inelastic, which one(s) will bring the car on the left to a complete halt?

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ConcepTest 9.24Crash Cars II ConcepTest 9.24 Crash Cars II If all three collisions below are totally inelastic, which one(s) will cause the most damage (in terms of lost energy)? A) I B) II C) III D) II and III E) all three

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ConcepTest 9.24Crash Cars II ConcepTest 9.24 Crash Cars II case III KE = 1/2 m v 2 case III largest velocity The car on the left loses the same KE in all 3 cases, but in case III, the car on the right loses the most KE because KE = 1/2 m v 2 and the car in case III has the largest velocity. If all three collisions below are totally inelastic, which one(s) will cause the most damage (in terms of lost energy)? A) I B) II C) III D) II and III E) all three

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ConcepTest 9.25Shut the Door! ConcepTest 9.25 Shut the Door! A) the superball B) the blob of clay C) it doesn’t matter -- they will be equally effective D) you are just too lazy to throw anything You are lying in bed and you want to shut your bedroom door. You have a superball and a blob of clay (both with the same mass) sitting next to you. Which one would be more effective to throw at your door to close it?

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Where is center of mass of a baseball bat located? A) at the midpoint B) closer to the thick end C) closer to the thin end (near handle) D) it depends on how heavy the bat is ConcepTest 9.26Baseball Bat ConcepTest 9.26 Baseball Bat

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Two equal-mass particles (A and B) are located at some distance from each other. Particle A is held stationary while B is moved away at speed v. What happens to the center of mass of the two-particle system? A) it does not move B) it moves away from A with speed v C) it moves toward A with speed v D) it moves away from A with speed 1/2 v E) it moves toward A with speed 1/2 v ConcepTest 9.27Motion of CM ConcepTest 9.27 Motion of CM

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ConcepTest 9.28Center of Mass ConcepTest 9.28 Center of Mass (1) X CM (2) A) higher B) lower C) at the same place D) there is no definable CM in this case The disk shown below in (1) clearly has its center of mass at the center. Suppose the disk is cut in half and the pieces arranged as shown in (2). Where is the center of mass of (2) as compared to (1) ?

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ConcepTest 9.28Center of Mass ConcepTest 9.28 Center of Mass (1) X X CM (2) The CM of each half is closer to the top of the semi-circle midpoint of the two semi-circle CM’s higher The CM of each half is closer to the top of the semi-circle than the bottom. The CM of the whole system is located at the midpoint of the two semi-circle CM’s, which is higher than the yellow line. A) higher B) lower C) at the same place D) there is no definable CM in this case CM The disk shown below in (1) clearly has its center of mass at the center. Suppose the disk is cut in half and the pieces arranged as shown in (2). Where is the center of mass of (2) as compared to (1) ?

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