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Homework Help: Inelastic collision problem

  1. Nov 29, 2003 #1
    A car weighing 900 kg is waiting at a stop sign. A car weighing 1200 kg hits the oter car. The cars move .76 m after the collision. The coefficient of friction between the sliding tires and the pavement is .92. I need to find the speed of the car right before the collision.
    I dont know what to use. Conservation of momentum doesnt work because I need at least one of the velocities.I can get the work done by friction fairly easily. But i can't get anywhere from there.
  2. jcsd
  3. Nov 29, 2003 #2

    Doc Al

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    Staff: Mentor

    Use conservation of energy (after the collision).
  4. Nov 29, 2003 #3
    but using conservation of energy wont work because the final velocity is zero so that sets one side to zero. leaving the other side just 1/2mv^2. im trying to find the v.
  5. Nov 29, 2003 #4


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    Science Advisor
    Gold Member

    It's implied that this is a totally inelastic collision (the cars stick together). You can use work-energy to find the velocity of the two cars immediately after the collision:

    .5*M*(v')2 = μMgd

    where M is the total mass (1200+900), v' is the velocity immediately after the collision, μ is the coefficient of friction, and d is the distance the cars slide.

    From conservation of momentum, you have mv = Mv', where m is the mass of the first car and v is what you're trying to find.
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