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Homework Help: Velocity of Car after Unloading Gravel

  1. Nov 27, 2007 #1
    1. A 10,000 kg railroad car is rolling at 2.00m/s when a 4000kg load of gravel is suddenly dropped in. What is the car’s speed just after the gravel is loaded?



    Relevant Equations

    W = 1/2 mv(f)^2 - 1/2 mv(i)^2

    3. The attempt at a solution[/b]

    W = 1/2 (14,000kg)v(f)^2 - 1/2(10,000)(2.0)^2


    I'm lost:$
     
  2. jcsd
  3. Nov 27, 2007 #2

    rock.freak667

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    Homework Helper

    The most I can guess is that consider momentum to be conserved..you can find the initial momentum and then you can formulate the final and they should be equal
     
  4. Nov 27, 2007 #3
    Pi= mi*vi
    = (10 000)(2.0)
    = 20 000

    Pi = Pf = 20 000

    Pf = mf*vf
    Vf = Pf/mf
    = 20 000/14 000
    = 1.42857
    = 1.43 m/s


    ???
     
  5. Nov 27, 2007 #4

    rock.freak667

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    Homework Helper

    Yes that would seem feasible as if it is heavier it would travel slower
     
  6. Nov 27, 2007 #5
    If you ignore friction and the initial velocity of the gravel then you can use conservation of momentum with ease.
    2*10000 = V * (4000 + 10000)

    I got 1.43m/s too. Explain the "???" Did you check it with the answer key and it's wrong or are you saying "is this right"? I'd say it's right.
     
  7. Nov 27, 2007 #6
    ??? meaning is it right...sorry for the confusion
     
  8. Mar 4, 2009 #7
    I did this same problem and got 1.43 m/s, but the books answer says .143 m/s. I was getting frustrated until I saw these posts. I guess the key is wrong.
     
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