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Homework Help: Help Momentum problems?

  1. Aug 14, 2007 #1
    Help!!Momentum problems?

    1. A girl jumps from a tree onto the ground. As she falls she speeds up.Explain in details how the law of conservation of momentum applies in this case? Can I consider the girl and the earth(the ground) as a system?? Why or why not??

    2. mass1*initial velocity1+mass2*initial velocity2=mass1*final velocty1+mass2*final velocity2

    3.I think that this the girl and the earth should be in one system. But I don't exactly know when is the initial status and final status. I am confused if i should consider the moment when the girl hits the ground to be final status or after the impact. Before the impact, the girl's v is downwards and the earth's v is upwards based on Newton's 3rd Law. So what is the direction of this system's momentum????How do I know????
  2. jcsd
  3. Aug 14, 2007 #2


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    I would say initial status, both velocities zero. Final status, just when the girl hits the ground - after she hits new (normal) forces start acting. You are correct to then deduce that if the girl is heading downward the earth is heading upward. But if total momentum was zero in the initial state for the system, your equation says it is zero in the final state.
  4. Aug 14, 2007 #3

    Thanks for Dick's explanation! :smile:But I still have some questions.What do you mean by both velocities zero? Does it mean the initial momentum of this system is zero? But how do you know or you just assume it? Is that normal which the earth acts on the girl is an external force??(I think it's an internal force, because it's the earth which is in the system acts)

    Can anyone explain the above to me????
  5. Aug 14, 2007 #4
    What do you mean by both velocities zero?

    Initially, the girl and the earth are moving at same speed (the speed with which the earth revolves around the sun).
    >So, relative to each other they both have 0 v.

    P=mv+MV = m0+M0=0 (so we know that)

    and, yes normal force is an internal force.
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