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Conservation of momentum, Elastic car collision.

  1. Apr 16, 2013 #1
    A 1689 kg car collides head on with a 2000 kg truck. The collision is elastic. If the velocity of the truck is 17km/h in the same directions as the car's initial velocity, what is the initial speed of the car in km/h?



    I'm getting an answer but doesn't seem to make sense.



    So I'm looking at the conservation of momentum equation to set this up.

    (1689-kg)(x)+(2000-kg)(0 km/h)=(1689-kg)(0)+(2000-kg)(17 km/h)

    Then: (1689)(x)=(34000) or x= 20.13

    I'm assuming that the final velocity of the car is 0 but it doesn't specifically say in the questions. Can I assume that since it's head on? Also it's a multiple choice question and the closest answer I can find is 19 km/h.
     
  2. jcsd
  3. Apr 16, 2013 #2

    haruspex

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    There's not enough information. Below, you assume the truck is initially stationary. Is that true? (Not what I would call 'head on'.)
    You should not. You are told the collision is elastic. What additional conservation law does that allow you to use?
     
  4. Apr 16, 2013 #3
    The Truck is parked so yes its initial is 0. Sorry I forgot to add that. Conservation of KE?
     
    Last edited: Apr 16, 2013
  5. Apr 17, 2013 #4

    haruspex

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    Yes.
     
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