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Impulse and Momentum: Substitution Trouble

  1. Mar 16, 2006 #1
    Ok, in this problem I am getting bogged down in the basic algebra part of it. I had one person explain it once but I still missed something.:frown:

    a 1055-kg van, stopped at a traffic light, is hit directly in the rear by a 715-kg car traveling with a velocity of +2.26 m/s. Assume that the transmission of the van is in neutral, the brakes are not being applied, and the collision is elastic. What is the final velocity of (a) the car and (b) the van?
    m1 = 715 kg
    m2 = 1055 kg
    vo1 = +2.25 m/s
    vo2 = 0 m/s
    vf1 = ?
    vf2 = ?
    http://www35.websamba.com/aerowenkitten/images/untitled.jpg [Broken]
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Mar 16, 2006 #2
    You know the initial masses and velocities and hence the initial momentum...from this you know the *relative* approach velocity before impact...and as the collision is elastic you also know the relative exit velocity...If you can form 2 equations using this info you should be able to spot a substitution easily...I don't think trying to solve in terms of KE is the best method
    Last edited: Mar 16, 2006
  4. Mar 16, 2006 #3
    Think of it this way.

    The initial momentum of the system (the car + the van) must equal the final momentum of the system (the car + the van).

    Treat velocities as vectors such that -> would be (+) and <- would be (-).

    That should help.
  5. Mar 16, 2006 #4
    AHHH... thank you! lol makes more sense that way!!
  6. Mar 16, 2006 #5
    No problem.
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