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Block hung from vertical spring

  1. Mar 30, 2017 #1
    1. The problem statement, all variables and given/known data
    A block is hung from a vertical spring. The spring stretches (h = .0650 m ) for a particular instant in time. Consider the earth, spring, and block to be in the system. If m = .865 kg and k = 125 N/m, find the change in the systems potential energy between the two times depicted.

    2. Relevant equations
    (1/2)ky^2(final) - (1/2)ky^2(initial)

    3. The attempt at a solution
    My book actually doesn't have an answer for this question. I assume the equation I wrote above is the one I use.

    Plugging in I got:

    (1/2)(125)(-.0650)^2 - (1/2)(125)(0) = .2640625

    does the answer I got seem correct to everybody?
     
  2. jcsd
  3. Mar 30, 2017 #2

    haruspex

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    The question statement is unclear. Is the block attached to the spring with the spring relaxed, and then released? The text implies a diagram, but you did not post one.
    It asks for the change in "potential energy", but you only calculated a change in elastic potential energy.
     
  4. Mar 30, 2017 #3
    Is there any gravitational potential energy involved? The question seems to imply that there should be.
     
  5. Mar 30, 2017 #4
    Sorry. Yes there is a diagram. It starts with a block at (0,0) and then the block goes to (0, -.0650 m)
     
  6. Mar 30, 2017 #5

    haruspex

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    So what is the total change in PE?
     
  7. Mar 31, 2017 #6
    Is it .2640625 Joules?
     
  8. Mar 31, 2017 #7

    haruspex

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    No, that's just the change in elastic potential energy. Reread posts #2 and #3.
     
  9. Mar 31, 2017 #8
    http://imgur.com/7OmNvFY

    Here is the full question. It is question number 26. The section its under is called "Elastic Potential Energy."

    Sorry I didn't make these clear in the OP
     
  10. Mar 31, 2017 #9

    haruspex

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    No, you had it right in the OP. It says to treat the Earth, spring and block as The System, and asks for the change in potential energy of The System. It does not restrict it to elastic potential energy. If it did, there would be no need to consider the Earth as part of The System.
     
  11. Mar 31, 2017 #10
    I see. So what I do now is add my answer in the op, with (Gravitational potential energy final - Gravitational potential energy final) to get total PE for the system?
     
  12. Mar 31, 2017 #11

    haruspex

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    Yes (except you didn't mean "final" twice).
     
  13. Mar 31, 2017 #12
    Wow I was sure I wrote (Gravitational potential energy final - Gravitational potential energy initial,) lol.

    So the key with this problem is understanding there is 2 sources of energy and to add these up to get the net potential energy, right?
     
  14. Mar 31, 2017 #13

    haruspex

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    It's that there are two forms of potential energy involved.
     
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