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Conservation of energy and momentum

  1. Mar 12, 2015 #1
    1. The problem statement, all variables and given/known data
    A girl of mass m1=60 kilograms springs from a trampoline with an initial upward velocity of vi=8.0meters per second. At height h=2.0 meters above the trampoline, the girl grabs a box of mass m2=15 kilograms.

    What is the speed of the girl after she grabs the box?

    2. Relevant equations
    Conservation of momentum

    3. The attempt at a solution
    In order to find this, I have to use conservation of energy to find the velocity right before she grabs the box, then use conservation of momentum. My question is where does the momentum go as the girl is rising? Isn't momentum supposed to be conserved? I know that internal forces conserve momentum, and the only force that acts on the girl is gravity. Is gravity not an internal force?
     
  2. jcsd
  3. Mar 12, 2015 #2
    Gravity is only internal if you consider the other agent in your system, namely the Earth. Since you're not, it's external
     
  4. Mar 13, 2015 #3

    Suraj M

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    I'm having a hard time finding out the exact time at which they are asking us to find the velocity.
    Does this mean that we cannot use conservation of angular momentum?
     
  5. Mar 13, 2015 #4

    jbriggs444

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    Immediately after the girl grabs the box.

    One could use conservation of angular momentum and arrive at the correct answer. But that would be a needlessly complicated approach. The problem can be solved in one dimension. Why invoke two?
     
  6. Mar 13, 2015 #5

    Suraj M

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    Would you get an accurate answer? because when she grabs it, she's at almost 5m/s, so after grabbing she would have moved by a certain height, shoúld we consider that too?or negligible?
     
  7. Mar 13, 2015 #6

    jbriggs444

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    Negligible. Almost all textbook problems are involve simplifying assumptions. Collisions take negligible time, ropes are massless, springs are perfect. In this case, "grabbing the box" amounts to an inelastic collision. Part of the what is being tested is the ability to recognize it as such.
     
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