Announcements

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Announcements Extra Credit

• Extra Credit for this six weeks
• Create an egg basket that can be dropped from 15
  feet and will not crack or break.
• Write a three paragraph paper that explains the
  physics concepts that goes along with it.
• Vocab that must be included is
• Collision - Force
• Velocity - Motion
• Impact - Momentum
• Terminal speed - Kinetic and Potential
  energy

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Announcements Extra Credit

• When will this project be submitted?
• The Tuesday before Thanksgiving
• What will it be worth?
• Replace a minor grade
• Or up to 20 points on a major grade

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Announcements

• Only 4 weeks left before finals!
• Watch your grades, this six weeks will go fast.
• Your next exam will be Tuesday Nov. 18th.
• You have until this Thursday to correct your
  work, power, and energy exam.

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Momentum and Impulse
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Lets start with everyday language

• What do you say when a sports team is on a roll?
• They may not have the lead but they may have
  ___________
• MOMENTUM
• A team that has momentum is hard to stop.

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What is Momentum?

• Momentum an object that is hard to stop
• Momentum p mv
• Units kgm/s
• mmass
• vvelocity
• Momentum is also a vector (it has direction)

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Lets practice

• A 1200 kg car drives west at 25 m/s for 3 hours.
  What is the cars momentum?
• Identify the variables
• 1200 kg mass
• -25m/s velocity
• 3 hours time
• P mv 1200 x -25 -30000 kg m/s

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How hard is it to stop a moving object?

• To stop an object, we have to apply a force
  over a period of time.
• This is called Impulse
• Impulse F?t Units Ns
• F force (N)
• ?t time elapsed (s)

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How hard is it to stop a moving object?

• Using Newtons 2nd Law we get
• F?t m?v
• Which means
• Impulse change in momentum

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Why does an egg break or not break?

• An egg dropped on a tile floor breaks, but an egg
  dropped on a pillow does not. Why?
• F?t m?v
• In both cases, m and ?v are the same.
• If ?t goes up, what happens to F, the force?
• Right! Force goes down. When dropped on a
  pillow, the egg starts to slow down as soon as it
  touches it. A pillow increases the time the egg
  takes to stops.

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Practice Problem

• A 57 gram tennis ball falls on a tile floor. The
  ball changes velocity from -1.2 m/s to 1.2 m/s
  in 0.02 s. What is the average force on the
  ball?
• Identify the variables
• Mass 57 g 0.057 kg
• ?velocity 1.2 (-1.2) 2.4 m/s
• Time 0.02 s
• using F?t m?v
• F x (0.02 s) (0.057
  kg)(2.4 m/s)
• F 6.8 N

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Car Crash

• Would you rather be in a head on collision
  with an identical car, traveling at the same
  speed as you, or a brick wall?
• Assume in both situations you come to a
  complete stop.
• Take a guess

http//techdigestuk.typepad.com/photos/uncategoriz
ed/car_crash.JPG
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Car Crash (cont.)

• Everyone should vote now
• Raise one finger if you think it is better to hit
  another car, two if its better to hit a wall and
  three if it doesnt matter.
• And the answer is..

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Car Crash (cont.)

• The answer is
• It Does Not Matter!
• Look at F?t m?v
• In both situations, ?t, m, and ?v are the same!
  The time it takes you to stop depends on your
  car, m is the mass of your car, and ?v depends
  on how fast you were initially traveling.

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Assignment

• Answer questions 2-5 on page 261.