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Question on conservation of momentum

  1. Oct 19, 2009 #1
    1. The problem statement, all variables and given/known data

    A 56-kg woman contestant on a reality television show is at rest at the south end of a horizontal 147-kg raft that is floating in crocodile-infested waters. She and the raft are initially at rest. She needs to jump from the raft to a platform that is several meters off the north end of the raft. She takes a running start. When she reaches the north end of the raft she is running at 4.6 m/s relative to the raft. At that instant, what is her velocity relative to the water?

    2. Relevant equations

    Psystem = Pwoman + Praft = 0

    Pwoman(f) - Pwoman(i) + Praft(f) - Praft(i) = 0

    3. The attempt at a solution

    I've split up the momentums into their each individual parts. The mass of the woman is 56kg, the raft is 147kg, and this is where I'm stuck. I believe that Pwoman(i) and Praft(i) are 0 because they both are initially at rest. Now I have Pwoman(f) + Praft(f) = 0. I don't know where to plug in her speed (Pwoman or Praft?), and either way, I would get a negative answer. Can someone help me figure out where to go from here?
  2. jcsd
  3. Oct 20, 2009 #2


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    Hi lilmul123! :smile:

    In questions about relative velocities, you really need two subscripts, not one.

    Call the woman j, the raft r, and the water w.

    [STRIKE]Then Pjw, for example, would be the momentum of the woman relative to the water.

    And Pjw = Pjr + Prw.[/STRIKE]

    Then Vjw, for example, would be the velocity of the woman relative to the water.

    And Vjw = Vjr + Vrw. :wink:
    Last edited: Oct 20, 2009
  4. Oct 20, 2009 #3
    How do I figure out the speed of the raft relative to the water?
  5. Oct 20, 2009 #4


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    (On second thoughts, my last post was rather confusing when it talked about momentum, so I've edited it :redface:)

    The question gives you Vjr.

    So find a physics equation relating Vjw and Vrw,

    and combine it with the geometry equation Vjw = Vjr + Vrw. :smile:
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