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Potential energy and springs

  1. Nov 12, 2008 #1
    1. The problem statement, all variables and given/known data

    A 700 g block is released from rest at height h0 above a vertical spring with spring constant k = 380 N/m and negligible mass. The block sticks to the spring and momentarily stops after compressing the spring 24.0 cm.

    (a) How much work is done by the block on the spring?

    (b) How much work is done by the spring on the block?

    (c) What is the value of h0?

    (d) If the block were released from height 2.00h0 above the spring, what would be the maximum compression of the spring?

    2. Relevant equations

    (KEi + PEi) + W = (KEf + PEf)
    W = F*d cos (thet)

    3. The attempt at a solution

    I plugged in .24m*380N/m (x * k) for the force the spring exerts, but I'm not quite sure what to do from here to get teh work for part a. any help is greatly appreciated, even just getting me started.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 12, 2008 #2


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    Gold Member

    The Work done/Energy associated with a spring is [tex]\frac{1}{2}kx^{2}[/tex] and I'm guessing all energy is conserved to Wblock = -Work spring

    Set Equal to potential energy to get h0

    Maximum compression is when Vblock = 0
  4. Nov 12, 2008 #3
    i see, i guess i had my equation wrong. Well that gets me A and B but i'm having trouble figuring out how to get the second part. if H is 0 where you drop the block, would that make me use -mgh and set that equal to final potential energy (which i don't think i know)?
  5. Nov 12, 2008 #4
    if i set the answer given from problems a and b equal to potential energy (mgh) i get some ridiculously small number. 10.994 / 700 / 9.8 = h = .00159 which seems impractical and is incorrect
    if i have Kei + Ui + W = kf + Uf where Kei, Ui, and kf all = 0 i'm left with W = Uf which gives me the same answer as above where 10.994 = mgh and a lot of division occurs!
  6. Nov 12, 2008 #5


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    I got h0 = 6.51m, I don't know where you got your answer.
  7. Nov 12, 2008 #6
    hmm h = 6.51 is incorrect according to the website (also tried -6.51).
    i've been reading up on this physics site just trying to understand these problems. I've kind of given up on finishing my hmwk by midnight, but i do have a midterm tomorrow and I want to understand how this works so i still want to do the problems heh.

    oh and thanks for tryin to help so far i do appreciate the input
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