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Homework Help: Conservation/Conversion of energy

  1. Nov 17, 2009 #1
    1. The problem statement, all variables and given/known data
    A mass of 4.5kg is dropped from a height of 3.3m above a spring. spring const is 25N/cm. How much is the spring compressed.

    2. Relevant equations

    F = 4.5*9.8 = 44.1

    PE = mgh = 4.5*3.3*9.8 = 145.53

    v = sqrt(2(9.8)(3.3)) = 8.04m/s

    3. The attempt at a solution

    The above are the few things I was able to grab out of the given information. The issue I'm having is plugging this into an equation to find the spring
  2. jcsd
  3. Nov 17, 2009 #2


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    Homework Helper

    Energy is conserved in this problem. If you write
    initial energy = final energy
    and then fill in the details of the kinds of energy involved you will be able to solve for the spring compression.
  4. Nov 17, 2009 #3
    so I get that mgh = 1/2mv^2+mgh?

    145.53 = .5(4.5)(8.04)^2+(4.5)(9.8)h


    I don't think I did it right
  5. Nov 17, 2009 #4
    Well I looked into a few of the formulas on the book and I get to a quadratic equation, but I'm still missing something here and I'm not sure what it is.


    356? That's not possible so any adive on this will be appreciated.

  6. Nov 18, 2009 #5
    Got it done. My equation above is right, the only problem was that I had to convert N/cm to N/m.

  7. Nov 18, 2009 #6


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    Homework Helper

    You didn't consider the spring!
    Initially you have the mgh. When the mass hits the spring, it comes to a stop and you only have spring energy ½kx².
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