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Vertical Elastic Collision Question

  1. Apr 18, 2009 #1
    1. The problem statement, all variables and given/known data

    A 70.0 kg person steps through the window of a burning building and drops to a rescue net held 8.00 m below. If the surface of the net is 1.40 m above the ground, what must be the value of the spring constant for the net so that the person just touches the ground when the net stretches down.

    2. Relevant equations

    How do I find the velocity of the man falling?

    3. The attempt at a solution

    Eg + Ee + Ek = Eg' + Ee' + Ek'

    Eg + Ek = Ee'

    mgh + .5mV2 = .5kx

    (70)(9.8)(9.4) + .5(70)V2 = 0.5k(1.4)2
     
  2. jcsd
  3. Apr 18, 2009 #2

    Doc Al

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    Staff: Mentor

    What's the man's initial speed? (He just steps through the window and falls.)
     
  4. Apr 18, 2009 #3
    Yea steps through and falls
     
  5. Apr 18, 2009 #4

    Doc Al

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    So what do you think his initial speed is?
     
  6. Apr 18, 2009 #5
    I got 12.2
     
  7. Apr 18, 2009 #6

    Doc Al

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    Is that the man's initial speed, when he just steps out the window? Or did you solve for the speed he would have at some later time?

    Hint: You should be able to state the man's initial speed immediately--no calculation needed.
     
  8. Apr 18, 2009 #7
    0 obviously I'm stating the velocity through the air
     
  9. Apr 18, 2009 #8

    Doc Al

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    All you need to solve this problem is the man's initial speed, not his speed at some later point. Compare initial energy to final energy.
     
  10. Apr 19, 2009 #9
    which energies kinetic or potential?
     
  11. Apr 19, 2009 #10

    Doc Al

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    Total energy: Kinetic + potential.
     
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