chapter 9 momentum - simon fraser university · two 1-kg stationary cue balls are struck by cue...

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

• Impulse

• Momentum

• The impulse-momentum theorem

• Conservation of momentum

• Inelastic collisions

Chapter 9MomentumTopics:

Sample question:Male rams butt heads at high speeds in a ritual to assert their dominance. How can the force of this collision be minimized so as to avoid damage to their brains?

Slide 9-1


Reading Quiz

1. Impulse isA. 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 9-2Answer: C


Reading Quiz

2. The total momentum of a system is conservedA. always.B. if no external forces act on the system. C. if no internal forces act on the system.D. never; momentum is only approximately

conserved.

Slide 9-3Answer: B


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 9-4Answer: B


Impulse

The force of the foot on the ball is an impulsive force.

Slide 9-5


Graphical Interpretation of Impulse

J = Impulse = area under the force curve

Slide 9-6


Momentum

Momentum is the product of an object’s mass and its velocity:

Slide 9-7

p = mv→ →

Momentum is a vector


The Impulse-Momentum Theorem

Impulse causes a change in momentum:

Slide 9-8

J =pf - pi = ∆p→ → →→


Example

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 9-9

pi = 0.5 kg × 10 m/s = 5 kg m/s

J ≈ 1

2(17 ms× 50 N)

≈ 425 N ms = 0.425 kg m/s

pf = pi +∆p

pi = mvi

pf = 5 kg m/s + 0.425 kg m/s = 5.425 kg m/s

vf =pfm

=5.425 kg m/s

0.5 kg

= pi + J


This is a one-dimensional problem...

= 10.85 m/s

vi = 10 m/sm = 0.5 kg


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?

Slide 9-10

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

Answer: C. Both balls receive the same impulse and have the same mass, so they will have the same final speed.


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.

Example

Slide 9-12


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?

Example

Slide 9-13


In terms of the initial and final total momenta:

The Law of Conservation of Momentum

In terms of components:

Slide 9-14

Pf = Pi→→


Example

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?

Slide 9-16


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.

Slide 9-17


Example

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 9-18


Example

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?

Slide 9-19

chapter 9 momentum - simon fraser university · two 1-kg stationary cue balls are struck by cue...

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