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Homework Help: Need help finding change in velocity and momentum

  1. Mar 28, 2013 #1
    An automobile of mass 1500 kg traveling at 24 m/s crashes into a similar parked automobile. The two automobiles remain joined together after the collision. Suppose that the collision last for 0.020s, and pavement with a coefficient of friction uk=0.63. What change of momentum and what change of speed does the friction force produce in the joined automobiles in the interval of 0.020 s ?

    Delta p=?
    Delta v=?

    for p I did 24m/s *1500kg=36000
    3600/12m/s=3000
    The Friction force = µ . m . g =0.63*9.81*3000=18541 N
    The change in momentum produced by this force = F . t =18541*0.02=371 kg*m/s
    delta p=36000-371=35629 kg*m/s

    to get v
    35629/3000=11.87m/s
    12m/s-11.87m/s=0.15 m/s
     
  2. jcsd
  3. Mar 28, 2013 #2
    What exactly do you need help with?
     
  4. Mar 28, 2013 #3
    the numbers that I input are apparently wrong.
     
  5. Mar 28, 2013 #4
    I am not sure what you mean by this. I have checked your computations and my results agree with yours.
     
  6. Mar 28, 2013 #5

    haruspex

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    Does the offending vehicle have its brakes on?
     
  7. Mar 28, 2013 #6
    it does not
     
  8. Mar 28, 2013 #7

    haruspex

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    So... what's the error in the equation I quoted?
     
  9. Mar 29, 2013 #8
    I should have used 1500kg for mass instead.
     
  10. Mar 29, 2013 #9

    haruspex

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    Exactly. Does it accept your answer now?
     
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