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Homework Help: Average resistance force to stop an object

  1. Apr 15, 2010 #1
    1. The problem statement, all variables and given/known data
    A 50.0 kg diver steps off a 10.0 m high diving board and drops straight down into the water. If the diver comes to rest 5.0 m below the surface of the water, determine the average resistance force exerted on the diver by the water.


    2. Relevant equations
    PEg=mgh
    KE=1/2 mv^2


    3. The attempt at a solution
    I did a similar question to this about a parachutist, but in that question there was a final velocity, in this one it seems like the final velocity would be 0? so im lost....
     
  2. jcsd
  3. Apr 15, 2010 #2
    What is the diver's velocity as he hits the water?
    What is his kinetic energy?
    What is the change in kinetic energy when the diver decelerates from that velocity to 0?
    What is the work done in decelerating the diver?
    What is the average resistive force on the diver as he decelerates?
     
  4. Apr 15, 2010 #3
    14.0m/s ?
    4900J ?
    wouldnt the change just be -4900J since the KEf would be 0?
    so work done is the same as change in kinetic energy.....
    what am i doing wrong?
     
  5. Apr 15, 2010 #4
    Work done is the change in kinetic energy, yes.

    Work done is also the average force multiplied by distance across which it is applied...
     
  6. Apr 15, 2010 #5
    so i divide the work done by 5.0m or 10.0m or 15.0m?
    the back of the book says the answer should be -1500N and i cant get that no matter how i do it
     
  7. Apr 15, 2010 #6
    I don't get that either. I get -1000N, taking g = 10 N/kg

    KE= GPE = mgh = 5000J

    All this is taken from the diver in the 5m of water: F = W/d = 5000/5 = 1000N.

    But I'm quite drunk at the moment so I might be wrong.
     
  8. Apr 15, 2010 #7
    haha alright well thanks for all the help.
     
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