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Homework Help: Problem 8.77 A Flea's Jump

  1. Aug 5, 2008 #1
    The resilin in the upper leg (coxa) of a flea has a force constant of about 26 N/m, and when the flea cocks its jumping legs, the resilin in each leg is stretched by approximately 0.10mm .

    Given that the flea has a mass of 0.55mg , and that two legs are used in a jump, estimate the maximum height a flea can attain by using the energy stored in the resilin. (Assume the resilin to be an ideal spring.)


    Thanks for the help!
  2. jcsd
  3. Aug 5, 2008 #2
    You need to make an attempt at a solution, it is not our job to do your homework for you. If you can show effort, than we can be of further assistance, if you are struggling.
  4. Aug 5, 2008 #3
    So you don't know the answer? I have plenty of work on my paper, but it's all chicken scratch.
  5. Aug 5, 2008 #4
    I could work out the answer, but the stickies for this forum give a clear structure for posting, which includes an attempt at the solution. What have you tried so far? What is going wrong?

  6. Aug 5, 2008 #5
    I'm trying to use hooke's law for the spring in the flea's legs.

    which would be F=kx
    F=(26N/m)(2x(for each leg)(0.0001m)
    I got a force of 0.0052

    I thought of using PE=mgh, the only equation I know of that would give me the height.
    I'm sure it's a simple problem I just don't know which equation to plug the given values into.


    1.16x10^-6 = 5.5x10^-6 (9.8)h
    h=2.15cm, but it's wrong.
  7. Aug 5, 2008 #6

    Doc Al

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    Staff: Mentor

    Hint: How much energy is stored in the flea's cocked legs?
  8. Aug 5, 2008 #7
    You're on the right track.

    You have F = kx. Work is force over a distance right? So if you find the integral of F = kx over the interval [0,.10], you can find the enegry stored in the flea's coked legs as Doc Al stated above.

    F = kx => W = 1/2kx^2
  9. Aug 5, 2008 #8
    1/2kx^2 = 1/2mv^2

    then plug into 1/2mv^2=mgh

    h= v^2/2g
  10. Aug 5, 2008 #9

    Doc Al

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    Staff: Mentor

    OK, but you can go directly from spring potential energy to gravitational potential energy. (No need to worry about kinetic energy.)
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