Transcript Momentum

Chapter 6 - Momentum
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Momentum
Impulse
Impulse Changes Momentum
Bouncing
Conservation of Momentum
Collisions
More Complicated Collisions
Physics 1100 - Spring 2012
Momentum
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Massive objects are harder to stop! (supertanker cuts engines
25 km from port)
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Momentum is mass times velocity
Momentum = mass * velocity
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Momentum is conserved in a closed system with no external
forces
Momentum (initial) = Momentum (final)
Physics 1100 - Spring 2012
Momentum and Impulse
• Impulse (J) = Force * Time Interval
• Impulses change momentum (p)
F x t = (ma)t = m x (at) = m x Dv
Impulse = F x t = D (mv)
Physics 1100 - Spring 2012
Momentum and Impulse
• Large change in momentum in a large time
Physics 1100 - Spring 2012
Momentum
• Large change in momentum in a small time
Physics 1100 - Spring 2012
Momentum
Physics 1100 - Spring 2012
Class Problem
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If a Mack truck and a Volkswagen
have a head-on collision, which
vehicle will experience the greater
impact force?
A)
B)
C)
D)
the Mack truck
the Volkswagen
both the same
... it depends on other factors
Physics 1100 - Spring 2012
Class Problem
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The answer is c:
Both will experience the same impact
force, in accord with Newton's 3rd Law.
The force that body A exerts on body B is
equal and opposite to the force that body
B exerts on body A. The effects of these
forces, however, are quite different—note
from Newton's 2nd Law
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Ftruck/Mtruck = atruck
Fcar/mcar = acar
So the car decelerates much more than
the massive truck.
Physics 1100 - Spring 2012
Class Problem
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Which would be more damaging;
driving into a massive concrete
wall, or driving at the same speed
into a head-on collision with an
identical car traveling toward you
at the same speed?
A) Wall
B) Head-on Collision
C) Same
Physics 1100 - Spring 2012
Class Problem answer
Both cases are equivalent, because either
way, your car rapidly decelerates to a dead
stop. The dead stop is easy to see when
hitting the wall, and a little thought will show
the same is true when hitting the car. If the
oncoming car were traveling slower, with
less momentum, you'd keep going after
collision with more "give," and less damage
(to you!). But if the oncoming car had more
momentum than you, it would keep going
and you'd snap into a sudden reverse with
greater damage. Identical cars at equal
speeds means equal momenta—zero
before, zero after collision.
Physics 1100 - Spring 2012
Conservation of Momentum
Physics 1100 - Spring 2012
Collisions
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Two types of collisions
– Elastic: Energy not dissipated out of kinetic energy
• Bouncy
– Inelastic: Some energy dissipated to other forms
• Sticky
Perfect elasticity unattainable (perpetual motion)
Physics 1100 - Spring 2012
Elastic Collision: Billiard Balls
• Whack stationary ball with identical ball moving at velocity vcue
8
To conserve both energy and momentum, cue ball stops dead,
and 8-ball takes off with vcue
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Momentum conservation: mvcue = mvcue, after + mv8-ball
Since cue ball can’t move through 8-ball, cue ball gets stopped.
Physics 1100 - Spring 2012
Real-World Collisions
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Is a superball elastic or inelastic?
– It bounces, so it’s not completely inelastic
– It doesn’t return to original height after bounce, so some
energy must be lost
Superball often bounces 80% original height
– Golf ball  65%
– Tennis ball  55%
– Baseball  30%
Depends also on surface, which can absorb some of the ball’s
energy
Physics 1100 - Spring 2012
Totally Inelastic Collisions
Physics 1100 - Spring 2012
Totally Inelastic Collision
Physics 1100 - Spring 2012
Inelastic Collisions
m = 5 kg
m = 1 kg
• Case 1: Vg = + 1 m/s, Vor = 0 m/s
• Case 2: Vg = + 1 m/s, Vor = - 4 m/s
Physics 1100 - Spring 2012
Class Problem
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A 100 kg ogre clobbers a dainty 50 kg figure skater while
trying to learn to ice-skate. If the ogre is moving at 6 m/s
before the collision, and the skater is at rest, at what speed
will the tangled pile be sliding afterwards?
A) 0 m/s
B) 3 m/s
C) 4 m/s
D) 5 m/s
E) 6 m/s
Physics 1100 - Spring 2012
More Complicated Collisions
Physics 1100 - Spring 2012
Class problems
1) The difference between impulse and impact force involves the
A) time the force acts.
B) mass and its effect on resisting a change in momentum.
C) difference between acceleration and velocity.
D) distance the force acts.
2) It is correct to say that impulse is equal to
A) the force multiplied by the distance the force acts.
B) momentum.
C) the change in momentum.
D) velocity multiplied by time
Physics 1100 - Spring 2012
Class Problems
3) A large heavy truck and a small baby carriage roll down a hill.
Neglecting friction, at the bottom of the hill, the baby carriage
will have a greater
A) acceleration.
B) speed.
C) momentum.
D) all of these
E) none of these
4) Compared to falling on a stone floor, a wine glass may not break
when it falls on a carpeted floor because the
A) carpeted floor provides a smaller impulse.
B) stopping time is shorter on the carpet.
C) stopping time is longer on the carpet.
D) carpet provides a smaller impulse and a longer time.
Physics 1100 - Spring 2012
Class Problems
5) A 1-N apple falls to the ground. The apple hits the ground with an
impact force of about
A) 4 N.
B) 9.8 N.
C) 1 N.
D) 2 N.
E) not enough information given to say
6) An astronaut , floating alone in outer space, throws a baseball. If
the ball floats away at a speed of 20 meters per second, the
astronaut will
A) move in the opposite direction, but at a lower speed.
B) move in the opposite direction but at a higher speed.
C) move in the opposite direction at a speed of 20 m/s.
D) not move as stated in any of the above choices.
Physics 1100 - Spring 2012