Chapter 9. Momentum. PowerPoint Lectures for College Physics: A Strategic Approach, Second Edition Pearson Education, Inc.

Transcription

1 Chapter 9 Momentum PowerPoint Lectures for College Physics: A Strategic Approach, Second Edition

2 9 Momentum Slide 9-2

3 Slide 9-3

4 Slide 9-4

5 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. Slide 9-5

6 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. Slide 9-6

7 Reading Quiz 2. The total momentum of a system is conserved A. always. B. if no external net force acts on the system. C. never; momentum is only approximately conserved. Slide 9-7

8 Answer 2. The total momentum of a system is conserved A. always. B. if no external net force acts on the system. C. never; momentum is only approximately conserved. Slide 9-8

9 Reading Quiz 3. In an inelastic collision, A. impulse is conserved. B. momentum is conserved. C. force is conserved. D. energy is conserved. E. elasticity is conserved. Slide 9-9

10 Answer 3. In an inelastic collision, A. impulse is conserved. B. momentum is conserved. C. force is conserved. D. energy is conserved. E. elasticity is conserved. Slide 9-10

11 Impulse The force of the foot on the ball is an impulsive force. Slide 9-11

12 Graphical Interpretation of Impulse J = Impulse = area under the force curve F avg t Slide 9-12

13 Slide 9-13

14 The Impulse-Momentum Theorem Impulse causes a change in momentum: Slide 9-15

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 Slide 9-16

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 Slide 9-17

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? We can model the curve as a triangle Slide 9-18

18 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? Or we can model the curve as a rectangle Slide 9-18

19 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? But let s model it as a triangle to do the problem Slide 9-18

20 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? Slide 9-18

21 Slide 9-19

22 Forces During a Collision We say momentum is conserved in a collision Slide 9-20

23 The Law of Conservation of Momentum In terms of the initial and final total momenta: In terms of components: Slide 9-21

24 Slide 9-22

25 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 kg hockey puck. After the collision, the puck s speed is 2.50 m/s. What is the stone s final velocity? Slide 9-23

26 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. Slide 9-24

27 Inelastic Collisions For now, we ll consider perfectly inelastic collisions: A perfectly inelastic collision results whenever the two objects move off at a common final velocity. Slide 9-25

28 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? Slide 9-26

29 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 (µ k = 0.20 for wood on wood). What was the bullet s speed? Slide 9-27

30 Angular Momentum Slide 9-28

31 Image from OpenStax College. Gyroscopic Effects: Vector Aspects of Angular Momentum.16 July 2014 <

32 Image from OpenStax College. Gyroscopic Effects: Vector Aspects of Angular Momentum.16 July 2014 <

33 Conservation of Angular Momentum Slide 9-28

34 Summary Slide 9-29

35 Summary Slide 9-30

36 Additional Questions Two cars, one heavier than the other, are used to test aspects of conservation of momentum on a test track. In the first test, the cars start from rest are driven in such a way that they each feel the same force from their engines for the same amount of time. Which car has the greater final momentum? A. The lighter car. B. The heavier car. C. They have the same momentum. Slide 9-31

37 Additional Questions In the second test, again starting from rest, the cars are driven so that they feel the same force from their engines, but this time they run for the same distance. Which car has the greater final momentum now? A. The lighter car. B. The heavier car. C. They have the same momentum. Slide 9-33

38 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 waterfilled barrels B. the crumple zone of her car (~1 m). Assume a constant acceleration. Slide 9-35

39 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? Slide 9-36