Transcript 09_LectureOutlines

Chapter 9
Momentum
Topics:
• Impulse
• Momentum
• The impulse-momentum
theorem
• Conservation of momentum
• Inelastic collisions
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?
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Slide 9-1
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 9-2
Answer
1. Impulse is
C. the area under the force curve in a
force-versus-time graph.
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Slide 9-3
Reading Quiz
2. The total momentum of a system is conserved
A. 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.
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Slide 9-4
Answer
2. The total momentum of a system is conserved
B. if no external forces act on the system.
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Slide 9-5
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.
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Slide 9-6
Answer
3. In an inelastic collision,
B. momentum is conserved.
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Slide 9-7
Impulse
The force of the foot
on the ball is an
impulsive force.
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Slide 9-8
Graphical Interpretation of Impulse
J = Impulse = area under
the force curve  Favg t
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Slide 9-9
Momentum
Momentum is the product of an object’s mass and its velocity:


p = mv
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Slide 9-10
The Impulse-Momentum Theorem
Impulse causes a change in momentum:




J =pf - pi = ∆p
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Slide 9-11
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?
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Slide 9-12
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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Slide 9-13
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?
C. Both balls have the same final speed
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Slide 9-14
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Slide 9-15
Example
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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Slide 9-16
Example
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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Slide 9-17
The Law of Conservation of Momentum
In terms of the initial and final total momenta:


Pf = P i
In terms of components:
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Slide 9-18
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Slide 9-19
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?
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Slide 9-20
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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Slide 9-21
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?
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Slide 9-22
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?
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Slide 9-23