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Spring elastic potential, removing mass

  1. Apr 30, 2017 #1
    1. The problem statement, all variables and given/known data
    5 identical masses M are suspended by a spring stretched a distance of L. If 3 of these masses are removed, what is the potential energy stored in the spring?

    2. Relevant diagram
    So L is the distance stretched with 5 masses.
    Let L2 be the distance stretched with 2 masses remaining.
    L2 < L since the spring would "shrink" if it had to hold up less mass.
    L2 = (2/5)L

    The potential energy is E = ½(k*x2)

    3. The attempt at a solution
    The answer which the book gives is (5/2)MgL
    Which would make sense if we are measuring the potential energy when the spring is holding 5 masses.
    Since F = 5Mg = kx, and x = L, so E = ½(k*x2) = ½(5MgL) = (5/2)MgL
    But isn't the question asking the potential energy when the spring is holding 2 masses?
    As I understand it, the potential energy would change:
    F = 2Mg = kx, and x = (2/5)L, so E = ½(k*x2) = ½(4/5MgL)=(2/5)MgL

    Which answer should it be?​
     
  2. jcsd
  3. Apr 30, 2017 #2

    Doc Al

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    Your answer sounds right to me. Where did you get this problem?
     
  4. Apr 30, 2017 #3
    http://imgur.com/a/2k2WV
    ih3k6IE.jpg

    Answer key says (3) is correct.
     
    Last edited by a moderator: Apr 30, 2017
  5. Apr 30, 2017 #4

    Doc Al

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    Could be a typo. Note that (2) cannot be correct -- the units are off. Only (1), (3), and (5) have proper units.
     
  6. Apr 30, 2017 #5

    haruspex

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    Yes, that is almost surely right, but I note that the question is not entirely clear. It does not say "3 masses are removed and the system allowed to come to equilibrium again". If the string is cut between the second and third mass from the top then, in that instant, the PE is still what it was with all 5. On the other hand, that would make the answer 3, not 2.
     
  7. Apr 30, 2017 #6

    Doc Al

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    Agreed. The question not clear.

    I thought of that, but dismissed it as being too sneaky without bothering to do the calculation. You know what, I think that's the "correct" interpretation after all -- it's a trick question! :wink:
     
  8. Apr 30, 2017 #7
    It indeed is a very tricky question! And also, very asinine! Because there is no point in saying "after 3 masses are removed" if we're still regarding the 5 mass system!

    Hope the AP Physics C exam won't be like that.
     
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