Momentum and Impulse

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Momentum and Impulse
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(No Transcript)
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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?

• An object with a lot of momentum is also hard to
  stop
• Momentum p mv
• Units kgm/s2
• 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, west velocity
• 3 hours time
• P mv 1200 x 25 30000 kg m/s2, west

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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.
• Videos
• smart car, suburban, bus, old vs. new, smart car
  70 mph, egg drop slow motion,
• Now 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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Egg Drop connection

• How did you use this in your egg drop?
• Which of these variables did you control?
• F?t m?v
• Which of them do you want to maximize, which do
  you want to minimize
• Note we are looking at the force on the egg.
• Therefore, m represents the egg mass, not the
  entire mass of the project

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Lets look at the calculations for your Egg Drop
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On Earth

• FE megg(g)

Impulse Force FIE (megg?v)/ ?t ?v Vbefore
collision Vafter collision ?t on Earth for the
collision 0.1 seconds
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On Jupiter

• FJ megg(j) Fall time is 0.02s at Jupiter's
  Gravity from 3.2 m j the acceleration on
  Jupiter due to gravity
• MJ 318 x ME

Impulse Force FIJ (?t) (megg?v) ?v Vbefore
collision Vafter collision Find ?t with only
the FIE and your Egg will survive.
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HINTS

• You need to calculate the following for Earth
• Use only the mass of the Egg now not the entire
  system.
• FIE (megg?v)/ ?t using your data.
• To find ?t with only the FIE and on Jupiter
• Find Vbefore collision by using j (acceleration
  on Jupiter) and the fall time on Jupiter of 0.02
  seconds in the Vfinal equation.
• Solve FIJ (?t) (megg?v) for ?t and substiture
  in the values form above using FIJ FIE to find
  ?t during the collision so that your Egg will
  survive.