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Chapter 9 Momentum © 2010 Pearson Education, Inc. PowerPoint® Lectures for College Physics: A Strategic Approach, Second Edition 9 Momentum © 2010 Pearson Education, Inc. Slide 9-2 © 2010 Pearson Education, Inc. Slide 9-3 © 2010 Pearson Education, Inc. Slide 9-4 Reading Quiz 1. Impulse is A. a force that is applied at a random time. B. a force that is applied very suddenly. C. the area under the force curve in a force-versus-time graph. D. the interval of time that a force lasts. © 2010 Pearson Education, Inc. Slide 9-5 Answer 1. Impulse is A. a force that is applied at a random time. B. a force that is applied very suddenly. C. the area under the force curve in a force-versus-time graph. D. the interval of time that a force lasts. © 2010 Pearson Education, Inc. Slide 9-6 Reading Quiz 2. The total momentum of a system is conserved A. B. C. D. always. if no external forces act on the system. if no internal forces act on the system. never; momentum is only approximately conserved. © 2010 Pearson Education, Inc. Slide 9-7 Answer 2. The total momentum of a system is conserved A. B. C. D. always. if no external forces act on the system. if no internal forces act on the system. never; momentum is only approximately conserved. © 2010 Pearson Education, Inc. Slide 9-8 Reading Quiz 3. In an inelastic collision, A. B. C. D. E. impulse is conserved. momentum is conserved. force is conserved. energy is conserved. elasticity is conserved. © 2010 Pearson Education, Inc. Slide 9-9 Answer 3. In an inelastic collision, A. B. C. D. E. impulse is conserved. momentum is conserved. force is conserved. energy is conserved. elasticity is conserved. © 2010 Pearson Education, Inc. Slide 9-10 Impulse The force of the foot on the ball is an impulsive force. © 2010 Pearson Education, Inc. Slide 9-11 Graphical Interpretation of Impulse J = Impulse = area under the force curve Favg t © 2010 Pearson Education, Inc. Slide 9-12 © 2010 Pearson Education, Inc. Slide 9-13 © 2010 Pearson Education, Inc. Slide 9-14 The Impulse-Momentum Theorem Impulse causes a change in momentum: © 2010 Pearson Education, Inc. Slide 9-15 Checking Understanding Two 1-kg stationary cue balls are struck by cue sticks. The cues exert the forces shown. Which ball has the greater final speed? A. Ball 1 B. Ball 2 C. Both balls have the same final speed © 2010 Pearson Education, Inc. Slide 9-16 Answer Two 1-kg stationary cue balls are struck by cue sticks. The cues exert the forces shown. Which ball has the greater final speed? A. Ball 1 B. Ball 2 C. Both balls have the same final speed © 2010 Pearson Education, Inc. Slide 9-17 Example Problem A 0.5 kg hockey puck slides to the right at 10 m/s. It is hit with a hockey stick that exerts the force shown. What is its approximate final speed? © 2010 Pearson Education, Inc. Slide 9-18 © 2010 Pearson Education, Inc. Slide 9-19 Forces During a Collision © 2010 Pearson Education, Inc. Slide 9-20 The Law of Conservation of Momentum In terms of the initial and final total momenta: In terms of components: © 2010 Pearson Education, Inc. Slide 9-21 © 2010 Pearson Education, Inc. Slide 9-22 Example Problem A curling stone, with a mass of 20.0 kg, slides across the ice at 1.50 m/s. It collides head on with a stationary 0.160-kg hockey puck. After the collision, the puck’s speed is 2.50 m/s. What is the stone’s final velocity? © 2010 Pearson Education, Inc. Slide 9-23 Rockets Rocket propulsion is an example of conservation of momentum: The rocket doesn’t push on the environment, as in propulsion. It pushes on the exhaust gas, and the exhaust gas pushes the rocket forward. Newton’s third law, but seen more easily from the perspective of conservation of momentum. © 2010 Pearson Education, Inc. Slide 9-24 Inelastic Collisions For now, we’ll consider perfectly inelastic collisions: A perfectly elastic collision results whenever the two objects move off at a common final velocity. © 2010 Pearson Education, Inc. Slide 9-25 Example Problem Jack stands at rest on a skateboard. The mass of Jack and the skateboard together is 75 kg. Ryan throws a 3.0 kg ball horizontally to the right at 4.0 m/s to Jack, who catches it. What is the final speed of Jack and the skateboard? © 2010 Pearson Education, Inc. Slide 9-26 Example Problem A 10 g bullet is fired into a 1.0 kg wood block, where it lodges. Subsequently, the block slides 4.0 m across a floor (µk = 0.20 for wood on wood). What was the bullet’s speed? © 2010 Pearson Education, Inc. Slide 9-27 Conservation of Angular Momentum © 2010 Pearson Education, Inc. Slide 9-28 Summary © 2010 Pearson Education, Inc. Slide 9-29 Summary © 2010 Pearson Education, Inc. Slide 9-30 Additional Questions In the demonstration, one car is heavier than the other, but both experience the same force and both run for the same time. Which car has the greater final momentum? A. The lighter car. B. The heavier car. C. They have the same momentum. © 2010 Pearson Education, Inc. Slide 9-31 Answer In the demonstration, one car is heavier than the other, but both experience the same force and both run for the same time. Which car has the greater final momentum? A. The lighter car. B. The heavier car. C. They have the same momentum. © 2010 Pearson Education, Inc. Slide 9-32 Additional Questions In the demonstration, one car is heavier than the other, but both experience the same force and both run for the same distance. Which car has the greater final momentum? A. The lighter car. B. The heavier car. C. They have the same momentum. © 2010 Pearson Education, Inc. Slide 9-33 Answer In the demonstration, one car is heavier than the other, but both experience the same force and both run for the same distance. Which car has the greater final momentum? A. The lighter car. B. The heavier car. C. They have the same momentum. © 2010 Pearson Education, Inc. Slide 9-34 Additional Example Problem A car traveling at 20 m/s crashes into a bridge abutment. Estimate the force on the driver if the driver is stopped by A. a 20-m-long row of water-filled barrels B. the crumple zone of her car (~1 m). Assume a constant acceleration. © 2010 Pearson Education, Inc. Slide 9-35 Additional Example Problem A 500 kg rocket sled is coasting at 20 m/s. It then turns on its rocket engines for 5.0 s, with a thrust of 1000 N. What is its final speed? © 2010 Pearson Education, Inc. Slide 9-36