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Chapter 9
Momentum
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PowerPoint® Lectures for
College Physics: A Strategic Approach, Second Edition
9 Momentum
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Slide 9-2
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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.
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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.
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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.
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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.
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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.
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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.
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Impulse
The force of the foot on
the ball is an impulsive
force.
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Graphical Interpretation of Impulse
J = Impulse = area under
the force curve  Favg t
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The Impulse-Momentum Theorem
Impulse causes a change in momentum:
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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
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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
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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?
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Forces During a Collision
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The Law of Conservation of Momentum
In terms of the initial and final total momenta:
In terms of components:
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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?
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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.
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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.
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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?
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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?
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Conservation of Angular Momentum
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Summary
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Summary
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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.
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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.
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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.
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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.
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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.
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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?
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