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Coefficient of Restitution problem

  1. Mar 10, 2017 #1
    Hello, I have this simple COR problem that I seem to be a bit stuck on.

    1. The problem statement, all variables and given/known data
    A 1.25 kg squash racquet swung at 37 m/s contacts a ball moving at 42 m/s in the opposite direction. Immediately after impact the racquet has a velocity of 25 m/s and the ball has a velocity of 50 m/s. What is the coefficient of restitution associated with the impact?

    2. Relevant equations
    I've been using e= (V1-V2)/(U1-U2)

    3. The attempt at a solution
    When I use this equation I get 5, but obviously that's wrong since it shouldn't be greater than 1.

    Can anyone point me in the right direction? Thanks so much
     
  2. jcsd
  3. Mar 10, 2017 #2

    PeroK

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    The operative word is direction!
     
  4. Mar 10, 2017 #3
    I sort of had a feeling this may be the case. Should I be using "U1+U2" instead of "U2-U1". Or am I way off?
     
  5. Mar 10, 2017 #4

    PeroK

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    Then depends on the direction of ##U_1## and ##U_2##. Are you using speeds or velocities?
     
  6. Mar 10, 2017 #5
    Velocities since it says 'in the opposite direction' I'm assuming.
     
  7. Mar 10, 2017 #6

    PeroK

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    So, what are the initial velocities of the racket and ball?
     
  8. Mar 10, 2017 #7
    They are 37 m/s (racket) and 42 m/s (ball)

    Ahhh I think I see what you may be getting at. Because it says the ball is moving in the opposite direction, should I be writing the equation like...

    50-25/37- -42
     
  9. Mar 10, 2017 #8

    PeroK

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    No, for velocities that can't be correct if they are in opposite directions.

    Only if you know why!
     
  10. Mar 10, 2017 #9
    Could you explain that statement. I thought the difference between speed and velocity is that velocity considers direction?
     
  11. Mar 10, 2017 #10

    PeroK

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    When you have a problem in 1D it's up to you to decide what direction is positive and what direction is negative. In this case I might decide that the racket is moving in the positive direction, which means that the velocity of the racket is ##37m/s##.

    The ball is moving in the opposite direction, so its velocity must be ##-42 m/s##.
     
  12. Mar 10, 2017 #11
    Ahhh of course. Thanks so much!
     
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