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An egg story

  1. Apr 29, 2010 #1
    Why??

    Why does the time affect velocity in this case not force (as in first case)?

    Thanks
     
  2. jcsd
  3. Apr 29, 2010 #2
    The first example is acceleration or total force. If the egg accelerates faster, as it does when it stops quickly on the metal plate, the force is greater.

    The tennis player example is Force over time. If the tennis player put a stronger force they could get the same velocity with a shorter contact time.
     
    Last edited: Apr 29, 2010
  4. May 2, 2010 #3
    I dun think this really helped
    I asked why does the linear momentum / impulse equal in both cases ?
    and

    Why does the time affect velocity in this case not force (as in first case)?
    I know that force is inversly proportional to time
    So do u have a better explanation?
    Thanks
     
  5. May 2, 2010 #4

    Borek

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    Staff: Mentor

    I would say by definition. How is impulse defined? What is momentum change in both cases?
     
  6. May 2, 2010 #5
    In the case of metallic plate
    The linear momentum of the egg before collision is mV1
    The linear momentum of the egg after collision is zero (since the terminal velocity is zero)

    In case of cushion:
    The linear momentum of the egg before collision is mV2
    The linear momentum of the egg after collision is zero (since the terminal velocity is zero)

    and according to the law of conservation of linear momentum :
    mV1 = mV2
    and force is inversely proportional to time when the change of linear momentum is constant

    but in the second case the change in linear momentum is not constant so time doesn't affect force

    right or wrong?
     
  7. May 2, 2010 #6

    Borek

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    Staff: Mentor

    Momentum of the egg before collision is in both cases identical - mV, so no need for V1 & V2.

    You have dodged my question about impulse definition and it is crucial for the answer.
     
  8. May 2, 2010 #7

    russ_watters

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    Staff: Mentor

    In the first case, the egg case (two cases, really), the velocity is provided by a fall under gravity, so it is equal.....it is essentially a starting condition of the problem, and not something to be investigated in the problem.

    In the tennis racket case, the velocity is variable because its the entire thing being investigated in the two cases.

    Basically, the two examples are examining opposite sides of the same coin: The first example examines how force can be altered by increasing the contact time and the second example examines how impulse can be altered by increasing contact time.
     
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