1

© 2010 Pearson Education, Inc.

PowerPoint® Lectures forCollege Physics: A Strategic Approach, Second Edition

Chapter 9

Momentum

© 2010 Pearson Education, Inc. Slide 9-2

9 Momentum

© 2010 Pearson Education, Inc. Slide 9-3

2

© 2010 Pearson Education, Inc. Slide 9-4

© 2010 Pearson Education, Inc.

Reading Quiz1. 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

© 2010 Pearson Education, Inc.

Answer1. 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

3

© 2010 Pearson Education, Inc.

Reading Quiz2. 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

© 2010 Pearson Education, Inc.

Answer2. 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

© 2010 Pearson Education, Inc.

Reading Quiz3. 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

4

© 2010 Pearson Education, Inc.

Answer3. 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

© 2010 Pearson Education, Inc.

Impulse

The force of the foot onthe ball is an impulsiveforce.

Slide 9-11

© 2010 Pearson Education, Inc.

Graphical Interpretation of Impulse

Slide 9-12

J = Impulse = area underthe force curve

5

© 2010 Pearson Education, Inc. Slide 9-13

© 2010 Pearson Education, Inc.

The Impulse-Momentum TheoremImpulse causes a change in momentum:

Slide 9-15

© 2010 Pearson Education, Inc.

Checking UnderstandingTwo 1-kg stationary cue balls are struck by cue sticks. The cuesexert the forces shown. Which ball has the greater final speed?

A. Ball 1B. Ball 2C. Both balls have the same final speed

Slide 9-16

6

© 2010 Pearson Education, Inc.

Two 1-kg stationary cue balls are struck by cue sticks. The cuesexert the forces shown. Which ball has the greater final speed?

Answer

A. Ball 1B. Ball 2C. Both balls have the same final speed

Slide 9-17

© 2010 Pearson Education, Inc.

Example ProblemA 0.5 kg hockey puck slides to the right at 10 m/s. It is hit with ahockey stick that exerts the force shown. What is its approximatefinal speed?

Slide 9-18

We can model the curve as a triangle

© 2010 Pearson Education, Inc.

Example ProblemA 0.5 kg hockey puck slides to the right at 10 m/s. It is hit with ahockey stick that exerts the force shown. What is its approximatefinal speed?

Slide 9-18

Or we can model the curve as a rectangle

7

© 2010 Pearson Education, Inc.

Example ProblemA 0.5 kg hockey puck slides to the right at 10 m/s. It is hit with ahockey stick that exerts the force shown. What is its approximatefinal speed?

Slide 9-18

But let’s model it as a triangleto do the problem

© 2010 Pearson Education, Inc.

Example ProblemA 0.5 kg hockey puck slides to the right at 10 m/s. It is hit with ahockey stick that exerts the force shown. What is its approximatefinal speed?

Slide 9-18

© 2010 Pearson Education, Inc. Slide 9-19

8

© 2010 Pearson Education, Inc.

Forces During a Collision

Slide 9-20

We say momentum is“conserved” in a collision

© 2010 Pearson Education, Inc.

In terms of the initial and final total momenta:

The Law of Conservation of Momentum

In terms of components:

Slide 9-21

© 2010 Pearson Education, Inc. Slide 9-22

9

© 2010 Pearson Education, Inc.

Example ProblemA curling stone, with a mass of 20.0 kg, slides across the ice at1.50 m/s. It collides head on with a stationary 0.160-kg hockeypuck. After the collision, the puck’s speed is 2.50 m/s. What isthe stone’s final velocity?

Slide 9-23

© 2010 Pearson Education, Inc.

RocketsRocket propulsion is an example of conservation of momentum:

The rocket doesn’t push on the environment, as in propulsion. It pusheson the exhaust gas, and the exhaust gas pushes the rocket forward.Newton’s third law, but seen more easily from the perspective ofconservation of momentum.

Slide 9-24

© 2010 Pearson Education, Inc.

Inelastic CollisionsFor now, we’ll consider perfectly inelastic collisions:

A perfectly inelastic collision results whenever the two objectsmove off at a common final velocity.

Slide 9-25

10

© 2010 Pearson Education, Inc.

Example ProblemJack stands at rest on a skateboard. The mass of Jack and theskateboard together is 75 kg. Ryan throws a 3.0 kg ballhorizontally 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

© 2010 Pearson Education, Inc.

A 10 g bullet is fired into a1.0 kg wood block, where itlodges. Subsequently, theblock slides 4.0 m across afloor (µk = 0.20 for wood onwood). What was thebullet’s speed?

Example Problem

Slide 9-27

© 2010 Pearson Education, Inc.

Angular Momentum

Slide 9-28

11

© 2010 Pearson Education, Inc.

Image from OpenStax College. Gyroscopic Effects: Vector Aspects of AngularMomentum.16 July 2014 < http://cnx.org/content/m42184/latest/?collection=col11406/1.7>

© 2010 Pearson Education, Inc.

Image from OpenStax College. Gyroscopic Effects: Vector Aspects of AngularMomentum.16 July 2014 < http://cnx.org/content/m42184/latest/?collection=col11406/1.7>

© 2010 Pearson Education, Inc.

Conservation of Angular Momentum

Slide 9-28

12

© 2010 Pearson Education, Inc.

Summary

Slide 9-29

© 2010 Pearson Education, Inc.

Summary

Slide 9-30

© 2010 Pearson Education, Inc.

Additional QuestionsTwo cars, one heavier than the other, are used to test aspectsof conservation of momentum on a test track. In the first test,the cars start from rest are driven in such a way that theyeach feel the same force from their engines for the sameamount 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

13

© 2010 Pearson Education, Inc.

Additional QuestionsIn the second test, again starting from rest, the cars aredriven so that they feel the same force from their engines, butthis time they run for the same distance. Which car has thegreater final momentum now?

A. The lighter car.

B. The heavier car.

C. They have the same momentum.

Slide 9-33

© 2010 Pearson Education, Inc.

A car traveling at 20 m/scrashes into a bridgeabutment. Estimate the forceon the driver if the driver isstopped by

A. a 20-m-long row of water-filled barrels

B. the crumple zone of hercar (~1 m). Assume aconstant acceleration.

Additional Example Problem

Slide 9-35

© 2010 Pearson Education, Inc.

A 500 kg rocket sled is coasting at 20 m/s. It then turns onits rocket engines for 5.0 s, with a thrust of 1000 N. What isits final speed?

Additional Example Problem

Slide 9-36