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Homework Help: Speed, Kinetic Energy, Force Problem.

  1. May 1, 2007 #1
    1. The problem statement, all variables and given/known data
    An 80 kg man jumps from a height of 3.00 m onto a platform mounted on springs. As the springs compress, the platform is pushed down a maximum distance of 0.200 m below its initial position, and then it rebounds. The platform and springs have negligible mass.

    a. what is the man's speed at the instant the platform is depressed 0.100 m

    b. if the man had just stepped gently onto the platform, how far would it have been pushed down?

    2. Relevant equations
    a = V/t
    v = d / t
    F = ma

    3. The attempt at a solution
    i've tried solving this one, and im stuck. heres what i did:

    a = V/t
    V = at

    V = d/t
    d/t = at

    t = (d/a)^-2 = 0.553 s

    V = 9.81(0.553) = 5.425 s

    F = ma = 80 kg (9.81) = 62.784 kJ

    and i dont know what to do next... please help me.
    Last edited: May 1, 2007
  2. jcsd
  3. May 1, 2007 #2


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    Gold Member

    Whats the actual question?
  4. May 1, 2007 #3
    i edited my previous post. i added the questions. my bad. :(
  5. May 1, 2007 #4


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    Science Advisor
    Homework Helper
    Gold Member

    Although this problem may be solved using kinematics and newton's laws, it is much easier to use energy methods. Are you familiar with the law of energy conservation?
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