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Momentum: I have the answer, now an explanation?

  1. Feb 17, 2009 #1


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    So, I know the "correct" answer to this problem, but it makes no sense to me at all. Why is the following the case?

    1. The problem statement, all variables and given/known data
    A railroad hopper car has mass 40,000 kg when empty and contains 50,000 kg of coal. As it coasts along the track at 4 m/s the hopper opens and steadily releases all the coal onto a platform below the rails over a period of 4 s.

    How fast does the car travel after all the coal is dumped?

    2. Relevant equations

    Momentum = Mass x Velocity

    3. The attempt at a solution
    40,000+ 50,000 = 90,000
    90,000 x 4 = 360,000

    360,000 / 40,000 = 9 m/s.
    This seems simple and obvious, yet the computer insists that the correct answer is 4 m/s. Why would this ever be....?
  2. jcsd
  3. Feb 17, 2009 #2


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    Homework Helper

    No friction or other force in the horizontal direction has been mentioned, so the car will continue in motion at its constant speed of 4 m/s. F = ma, F is zero so "a" is zero.
  4. Feb 17, 2009 #3


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    Homework Helper

    Why do you think it is the right answer?

    Are you saying that as soon as each lump of coal was dropped from the car it left its momentum in the car?
  5. Feb 17, 2009 #4


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    I obviously wasn't considering that the coal continues moving as it is dropped, and doesn't leave it's momentum behind, which is the premise I was operating under. Many thanks!
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