Momentum Review

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Momentum Review
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Momentum

• Momentum
• a measure of how hard it is to stop a moving
  object.
• The direction of the momentum is the same as that
  of the velocity.
• p m v
• where p is momentum, m is mass in kg, and v is
  velocity in m/s

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Impulse

• a force exerted over a time interval symbol is J
  and SI unit is N sec
• J F t
• where J is impulse, F is force, and t is time in
  seconds

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Impulse Momentum Theorem

• F(t) p2 p1
• The impulse on an object is equal to the change
  in momentum that it causes. If the force is
  constant, the impulse is simply the product of
  the force times the time interval over which it
  acts.

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Law of conservation of momentum

• The law of conservation of momentum states that,
  in the absences of an external force, the
  momentum of a system remains unchanged.
• pi pf

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Law of conservation of momentum

• For example, if you push on the dashboard of a
  car, you dont increase the cars momentum.
• The molecular forces within a basketball have no
  effect on the momentum of the basketball.
• To change the momentum of the car or basketball
  will require an external force.
• If there is no outside force, no change in
  momentum is possible.

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Law of conservation of momentum

• Consider a cannon being fired. The force on the
  cannonball inside the cannon barrel is equal and
  opposite to the force causing the cannon to
  recoil. These forces are internal to the system,
  so they dont change the momentum of the cannon.
• Before the firing, the net momentum is zero, and
  after the firing, the net momentum is zero.

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Law of conservation of momentum

• What would happen if you stood on a skateboard
  and threw a ball forward? What would happen to
  you?
• Which of Newton's laws states this?
• What would happen if you did the same experiment
  but didnt let the ball go? What direction would
  you move?
• Why did you move in the above experiment?

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Formula

• Conserved Momentum

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types of collisions

• 1) Elastic collision
• total momentum and total energy is conserved
  elastic collisions only occur on the sub-atomic
  particle level
• 2) Inelastic collision
• total momentum is conserved real-life collisions
  are all inelastic

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Vocabulary

• System
• a term that describes a collection of objects
• Closed system
• mass is constant
• Open system
• mass is not constant
• Isolated system
• one in which no external force acts
• Also, when the net external force on a closed
  system is zero.

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Collisions

• Whenever objects collide in the absence of
  external forces, the net momentum of both objects
  before the collision equals the net momentum of
  both objects after the collision.
• Net momentum (before) Net momentum (after)

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Collisions

• (net mv) before net(mv) after.
• Example Suppose two freight train cars are of
  equal mass, and that one car moves at 4 m/s
  toward the other car that is at rest.
• Can you predict the velocity of the coupled cars
  after the impact?

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• M(4 m/s) m (0 m/s) 2m (v)
• V 2 m/s
• Since twice as much mass is moving after the
  collision, can you see that the velocity after
  must be one half of 4 m/s in the same direction
  as the velocity before the collision.

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• Consider a 6 kg Kevin that swims toward and
  swallows a 2 kg Jose that is at rest. If the
  Kevin swims at 1 m/s, what is his velocity
  immediately after he eats Jose?
• Remember Net Momentum before lunch net momentum
  after lunch.
• 6 kg (1 m/s) 2 kg (0 m/s) 6 kg 2 kg (v)
• 6 kg m/s 8 kg (v)
• V ¾ m/s

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Problem 1

• A 2275 kg car going 28 m/s rear-ends an 875 kg
  compact car going 16 m/s on the ice in the same
  direction. The two cars stick together. How
  fast does the wreckage move immediately after the
  collision?
• Write down what you know.

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• M 1 2275 kg
• V 1 28 m/s
• M 2 875 kg
• V 2 16 m/s
• 2275 ( 28) 875 (16) 2275 kg 875 (v)
• V 25 m/s