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Homework Help: Momentum and velocity

  1. Mar 28, 2010 #1
    1. The problem statement, all variables and given/known data

    A railroad handcar is moving along straight, frictionless tracks with negligible air resistance. In the following cases, the car initially has a total mass (car and contents) of 160 kg and is traveling east with a velocity of magnitude 4.70 m/s. Find the final velocity of the car in each case, assuming that the handcar does not leave the tracks.

    An object with a mass of 28.0 kg is thrown backward out of the car with a velocity of 4.70 m/s relative to the initial motion of the car.

    2. Relevant equations


    3. The attempt at a solution

    the momentum of the system is

    p = (160+28)(4.7) = 883.6


    883.6 = 160(vf) + 28(-4.7+4.7)

    883.6 = 160vf

    vf = 5.52 m/s

    Where did i got wrong? :(

    Thank you for any help
  2. jcsd
  3. Mar 28, 2010 #2


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    Homework Helper

    The total mass of the cart is 160kg, so your total momentum would be 160*4.7.

    After it is thrown out, it would lose 28 kg. So the final mass is 160-28.
  4. Mar 28, 2010 #3
    Thank you :)

    i was reading the question wrong haha i always seem to do that.
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