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Homework Help: Resistance Force

  1. Nov 17, 2009 #1
    1. The problem statement, all variables and given/known data

    A 49.0 kg diver steps off a 12.0 m high diving board and drops straight down in to the water. If the diver comes to rest 5.0 below the surface of the water, determine the average resistance force exerted on the diver by the water.

    2. Relevant equations



    3. The attempt at a solution

  2. jcsd
  3. Nov 17, 2009 #2
    What is the potential energy of the diver 12m above the water? The water will have to do an equivalent amount of work over a distance of 5m to stop the diver.

    Remember: W = F*d (where W = work, F is force, d is distance)
  4. Nov 18, 2009 #3
    So I should find the PE of the diver at 12m?

    The question asks to determine the average resistance force exerted on the diver by the water.

    So I would use W=F*D?

    How do I put this all together now?
  5. Nov 18, 2009 #4
    That's correct. Don't forget the unit of energy: 5768.28 J (Joules)

    Now that potential energy will be converted to kinetic energy as the diver falls. At the surface of the water, the diver will have 5768.28 J of KE.

    As I stated before, the water will have to do an equivalent amount of Work (5768.28 J) to stop the diver- both work and energy are measured in Joules. Knowing that this happens over a distance of 5 meters, you can plug the known values into the Work formula and and solve for F:
    W = F*D

    F = W/D
  6. Nov 18, 2009 #5
    I don't know if i'm just not doing something right or what it is but, i calculated PE:

    PE=5768.28 J

    Then you told me I should plug this in for work:

    F=1135.65 N

    unfortunately this is not the correct answer... Any ideas what it could be??
  7. Nov 18, 2009 #6
    I'm getting 1153.65 N. I'm sure your 1135 is a typo. What's given as the correct answer?
  8. Nov 18, 2009 #7
    Yes that was a typo, but I don't know the correct answer I have to submit to a website and it only lets me know whether my answer is correct or not and it is telling me that 1153.65 N is not correct. Are there any other approaches to this problem?
  9. Nov 18, 2009 #8
    Yes. I made a mistake. We need to consider the 5m below the water as well as the 12m in the PE calculation. Use 17m since that is the actual height the diver falls.

    Sorry about that. Stupid mistake...
  10. Nov 18, 2009 #9
    Its 1634.34 N

    Thanks so much for the help.
  11. Nov 18, 2009 #10
    Your welcome. I apparently need a little practice myself. :D
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