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Finding quantity of energy stored in a spring

  1. Mar 2, 2014 #1
    1. The problem statement, all variables and given/known data
    Hey guys, I'm having some trouble finding the exact quantity of energy storied in this spring problem.
    It's like a problem with parts A and B, part A says "Determine the amount of work required to stretch the spring from 0 to 20 cm. I found that to be 125J. For Part B, it says "Suppose that this spring was stretched 7.753 cm. Find the exact quantity of energy stored in the spring.


    2. Relevant equations
    I think that the equation that i would use is the one for elastic energy, Ee= 1/2kx^2


    3. The attempt at a solution
    So far i have Ee= 1/2kx^2

    Ee= 1/2k(7.753)^2
    I'm not sure if i'm doing this right, or what to do next.

    Thanks
     
  2. jcsd
  3. Mar 2, 2014 #2

    Doc Al

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    Staff: Mentor

    How did you solve part A?
     
  4. Mar 2, 2014 #3
    For part A, it had a diagram and the displacement was 20 cm and the force was 12.5N so i calculated 20*12.5/2= 125J
     
  5. Mar 2, 2014 #4

    Doc Al

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    Staff: Mentor

    I assume that the 12.5 N force was the force required to stretch the spring 20 cm. So what's the spring constant?

    Careful with units! (Convert distances to standard units: m, not cm.)
     
  6. Mar 2, 2014 #5
    I divided 12.5 by 20 and got 0.625, but should i have converted before dividing?
     
  7. Mar 2, 2014 #6

    Doc Al

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    Staff: Mentor

    Yes. You want k in N/m, not N/cm.

    (Note that you'll have to revise your answer for part A.)
     
  8. Mar 2, 2014 #7
    Ok, i think i got the right answer.
    Ee= 1/2 (625)(0.0078)^2
    Ee= 1/2(625)(0.00006)
    Ee= 0.01875J

    Thank for all the help!
     
  9. Mar 2, 2014 #8

    Doc Al

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    Staff: Mentor

    Careful! (But you're almost there.)

    Check your values for k and for the amount of stretch.
     
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