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Homework Help: Electricity and SHM

  1. Apr 5, 2015 #1
    1. The problem statement, all variables and given/known data
    In the figure mA = mB = 1kg . Block A is neutral while qB = -1C, sizes of A and B are negligible. B is released from rest at a distance 1.8m from A . Initially springs is neither compressed nor elongated.
    View the figure above.
    Equilibrium position of the combined mass is at x = ....... m

    Note: Collision between A and B is perfectly inelastic.

    2. Relevant equations

    3. The attempt at a solution
    a= 10m/s (of block B using qE/m)
    v = root(2as) = 6m/s
    mv = 2mv'
    v' = 3m/s (combined velocity of the system)

    1/2 mv^2 = 1/2 kx^2 - qEx
    9 = 9x^2 - 10x
    x = -0.588
    The answer is -5/9, which is quite close to my answer but not exactly.
    Can someone just tell me if what I've done is right?
  2. jcsd
  3. Apr 5, 2015 #2

    rude man

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    Your equation makes no sense. It is not even dimensionally correct. qE is a force, the other terms are energy.
    At equilibrium the electrostatic force must be equal and opposite to the spring force. That gives x = -5/9 exactly.
  4. Apr 5, 2015 #3
    He put qEx, not qE, which is an attempt to add in a work contribution i. E. Fd.

    Although your method seems correct, just didn't want to give him the wrong idea
  5. Apr 5, 2015 #4

    rude man

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    OK, I thought he meant E-sub-x. Still doesn't make sense, the equation I mean.
    Well, my method has to be correct. May be some other way is too.
  6. Apr 5, 2015 #5
    Okay thanks, got it!
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