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Finding final velocity from a fall

  1. Dec 15, 2006 #1

    lzh

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    I'm having trouble with this problem:
    So for this problem i used F=ma:
    79g-88=79a
    a=8.6861m/s^2
    this is the correct acceleration i believe and to find final velocity i used:
    Vf^2=Vi^2+2ad where d is displacement
    Vf^2=2(8.6861)(25)
    vf=20.85m/s
    I'm using utexas's webhw service, and the answer above was not correct, can someone tell me what i'm doing wrong?
     
    Last edited: Dec 15, 2006
  2. jcsd
  3. Dec 15, 2006 #2
    not 100% sure on this but i think:
    the 79g-88N=79a is wrong the units dont cancel out properly, you know the 88N upward on the person what's the force he excerts downard?
    with those two figures you can use F=ma knowing F and m to determine the actual a, and plug that into the Vf^2=Vi^2+2ad equation
     
  4. Dec 15, 2006 #3
    I solved the problem by calculating the amount of downward force one the person; mainly his weight minus the resistance. Then I used that F in the work formula; W=FD. D would be the 25m. I got the W in Jules so I let that W = KE, which is kinetic Energy. so, W = MV^2. and solved for the desired quantity.
    Same ans. So I suppose you’re ans is right.
     
  5. Dec 15, 2006 #4

    lzh

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    79g is his weight, which is in N, which cancels out
    but it isn't right...
     
  6. Dec 15, 2006 #5
    I'm not sure wat u mean. But the unites do cancel out propely:
    (790N-88N)/79 = a.
     
  7. Dec 15, 2006 #6
    well N is kg*m/s^2 79kg is just the weight you're missing the acceleration (gravity)
     
  8. Dec 15, 2006 #7
    Typo:
    *79kg.
     
  9. Dec 15, 2006 #8

    lzh

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    yeah g as in acceleration due to gravity
     
  10. Dec 15, 2006 #9
    sry was distracted >.< i get the same answer as you.
    are you providing the required number of significant digits?
     
  11. Dec 15, 2006 #10

    lzh

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    rounding and sig figs doesn;t matter
     
  12. Dec 15, 2006 #11
    out of curiosity; what kind of deceleration is applied form falling on the net for a safe landing. Too much would cause the person to recoil up. too little would cause he/she to hit the ground.
     
  13. Dec 16, 2006 #12

    Doc Al

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    Staff: Mentor

    Your method and answer look correct to me.
     
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