Glencoe Physics Chapter 9

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Glencoe Physics Chapter 9

• Momentum and Its Conservation

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Air Bags

• How exactly does an airbag prevent serious injury
  in an auto accident?

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Baseball

• Why are baseball players encouraged to follow
  through when hitting the ball with a bat?

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Why does a cannon move backward when firing?
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Highway safety

• What can we do to make highways safer for people?

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Common usage of the term Momentum

• A team that has momentum is on the move and is
  going to take some effort to stop.
• Going into the final weeks, the St. Louis
  Cardinals have the momentum."
• The headlines declare Pittsburgh Steelers
  Gaining Momentum."
• A team that has a lot of momentum is really on
  the move and is going to be hard to stop.

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Momentum (from latin, meaning movement)

• Momentum is a physics term it refers to the
  quantity of motion that an object has.
• Refers to how hard something is to stop.

• 1330 Jean Buridan the true measure of an
  objects motion
• 1600s Rene Descartes quantity of motion
• Galileo Momento
• Newton Momentum

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Which is probably harder to stop????? Why?
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Momentum Defined
Yep.thats p for Momentum!!!
p m v
p momentum m mass v velocity
Unit must be kg m/s
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• So momentum of an object depends on the amount of
  stuff moving (mass), and the velocity at which
  the stuff is moving.
• Question? Under what conditions of motion would a
  semi-truck and a roller skate have the same
  momentum?

Road Train in the Australian Outback
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Equivalent Momenta
Car m 1800 kg v 80 m /s p
1.44 105 kg m /s
Bus m 9000 kg v 16 m /s p
1.44 105 kg m /s
Train m 3.6 104 kg v 4 m /s
p 1.44 105 kg m /s
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Momentum Facts

• Momentum is a vector quantity
• Velocity and momentum vectors point in the same
  direction
• SI unit is the kgm/s ( no special name)
• Momentum is a conserved quantity
• Momentum is directly proportional to mass and
  velocity of object

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Momentum Facts

• A net force is needed to change a particular
  objects momentum (a change in velocity)
• Something big and slow can have the same momentum
  of something small and fast

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Momentum Examples
Now, find the vertical and horizontal components
of the momentumgtgtgtgtgt
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Sample problems

• Find the magnitude of the momentum of a car of
  mass 725 kg moving at a velocity of 100. km/h.

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Sample problems

• Calculate the velocity of a car that has a mass
  of 2175 kg and the same momentum as the previous
  car.

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Impulse

• In order to change the momentum of an object, a
  force must be applied
• Gives an alternative statement of Newtons second
  law..
• Watch as a little mathematical magic is performed!

Magic???
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When an impulse (force applied over a period of
time) is applied, a change in momentum will occur
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Big Bertha 400-ton barrel
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Shells weighed 7 tons!!!