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Homework Help: Force done by the spring if a weight is attached

  1. Oct 13, 2011 #1
    1. The problem statement, all variables and given/known data

    What is the force done by the spring if an object of mass m is attached.

    2. Relevant equations
    F=ma
    F=-kx
    Fg=mg


    3. The attempt at a solution

    Sum F = 0
    mg-kx=0
    mg-Fs=0
    Fs=mg

    Therefore the force done by the spring is mg? is the +/- right? for direction? Is the force right?
     
  2. jcsd
  3. Oct 13, 2011 #2
    Properly think about it:

    The mass is hanging from the spring. I assume that when it is observed, it can be considered at rest.
    If it's at rest, there are only two forces acting on the mass: gravity and the spring force. Since the mass is at rest, all forces must cancel eachother.
    .
    Since both forces are acting in opposite directions, they shouldn't have the same sign.

    You stated that Sum F = 0. Therefore, Fs + Fg = 0
     
  4. Oct 13, 2011 #3
    Ok so then Fs=-Fg=-mg.
    Just change the sign of my answer?
     
  5. Oct 13, 2011 #4
  6. Oct 13, 2011 #5
    ok thank you very much!! i thought the sign was a bit messed up.
     
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