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Find the initial velocity of truck before collision

  1. Mar 4, 2015 #1
    Question: A car of mass 1500kg collides head-on with a truck of mass 9000kg. After the collision, the wreckage is at rest. The speed of the car just before the collision was 30m/s. What was the speed of the truck just before the collision?

    Relevant Equation(s):

    m1v1+m2v2 = m1v1'+m2v2' (?)

    My Attempt:

    (1500kg)(30m/s)+(9000kg)( ___m/s) = (1500kg)(0m/s)+(9000kg)( 0m/s)
    (1500kg)(30m/s)+(9000kg)( ___m/s) = 0
    (45000 kg*m/s)+(9000kg)(___m/s) = 0

    Problem: It's 1 in the morning and I'm not understanding what I'm doing wrong. Am I using the right equation? Do I need to set up west and east on the x-axis??
     
  2. jcsd
  3. Mar 4, 2015 #2

    Orodruin

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    Why do you think you are doing something wrong? You will get a negative answer, but that only tells you the truck is moving in the opposite direction of the car.
     
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