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Homework Help: Momentum Question

  1. Dec 28, 2007 #1
    1. The problem statement, all variables and given/known data
    A 82kg girl is coasting along the floor on a large 2.0kg skateboard. If she is moving 1.8 m/s when a 46kg boy jumps on to the same skateboard, what is the speed of the skateboard immediately after the boy jumps on it?

    2. Relevant equations

    Sum of Initial Momentum = Sum of Final Momentum

    3. The attempt at a solution

    This question looked pretty easy.. but Im sure Im doing something wrong... here's what I did so far:
    (84kg)(1.8m/s) + (130kg)(0m/s) = (84kg + 130kg)Vf
  2. jcsd
  3. Dec 28, 2007 #2
    Sorry.. forgot the last part..
    I got vf=0.7065m/s

    Im not sure if this is right or not.. it SEEMS like an okay speed.. but I havn't done this type of question before.. so just want to be sure. Thanks!
  4. Dec 28, 2007 #3

    Gib Z

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    You should only be looking at the initial and final momentums of the skateboard (and what ever is on it).

    So it starts off with a total mass of 84 kg, going at 1.8 m/s. At the end, It has the mass of a 46 kg boy, so the total mass is 130 kg, and v_1 m/s is what we are trying to find. Equating both sides and making v_1 the subject;

    [tex] 84 kg \cdot 1.8 ms^{-1} = 130 kg \cdot v_1 ms^{-1}[/tex]

    [tex] v_1 = \frac{84 \cdot 1.8}{130}[/tex]

    I can already tell thats larger than one, so recheck your answer =]
  5. Dec 29, 2007 #4
    Hmm.. that makes sense.. but the answer to that would be 1.163m/s. The answer is 0.97m/s. I don't know how to get to that..
    Last edited: Dec 29, 2007
  6. Dec 29, 2007 #5

    Gib Z

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    Well, That is somewhat odd...I think it's an error in the textbook, but it has been a while since I've studied Physics so I recommend you wait for someone who has more knowledge in this area to confirm.
  7. Dec 29, 2007 #6


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    unless there are something else in the problem we have not addressed, otherwise I believe Gib Z's correct.
  8. Dec 31, 2007 #7
    Yeah.. more than likely it is.. thanks for the help!
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