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Homework Help: A really confusing question on potential energy, please help

  1. Mar 25, 2016 #1
    1. The problem statement, all variables and given/known data
    A 975 g block is released from rest at height h0 above a vertical spring with spring constant k = 410 N/m and negligible mass. The block sticks to the spring and momentarily stops after compressing the spring 24.2 cm. How much work is done (a) by the block on the spring and (b) by the spring on the block? (c) What is the value of h0? (d) If the block were released from height 4h0 above the spring, what would be the maximum compression of the spring?

    2. Relevant equations
    Mechanical Energy: Emec=K+U
    Conservation of Emec: K2+U2=K1+U1
    Elastic Potential Energy: U(x)=0.5mk^2
    Gravitational Potential Energy: U=mgh
    3. The attempt at a solution
    Part a&b are fairly straight forward Ive got them right,
    my attempt for part c was to set the "momentarily stop" as 0 for the height, and thus have
    mgh0 = 0.5kx^2
    and h0=1.256475144... but i was marked incorrect for this value.
    so is my answer to part d, x=0.484m wrong.
  2. jcsd
  3. Mar 26, 2016 #2
    i think you have missed a point in energy conservation - the change in P.E. =mg (ho+x) as the mass moves down during comprssion of the spring . so your energy gets missing by part mgx-ho should be less.
  4. Mar 26, 2016 #3


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    Check where h0 is measured from.
  5. Mar 31, 2016 #4
    Thanks a lot mate
  6. Mar 31, 2016 #5
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