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Homework Help: What am I doing wrong?

  1. May 30, 2005 #1
    Here's the prob:

    A 5.00kg block carrying a charge Q = 40.0 μC is attached to a spring for which k = 100 N/m. The block lies on a frictionless horizontal track, and the system in immursed in a uniform electric field of magnitude E = 5.00 x 10^5 V/m. a) The block is released from rest when the spring is unstretched (at x = 0). By what maximun amount does the spring expand? b) what is the equilibrium position of the block?

    What I did:


    1 V/M = 1 N/C

    F = qE (in electric field) = (40.0 x 10^-6 C)(5.0 x 10^5 N/C) = 20 N

    F = -kx (Hooke's Law) so:

    20N = -(100 N/M)x
    x = 20N / -(100N/M)
    x = -0.2m

    But this is wrong! Where am I messing up.

    b). I haven't attemped yet. How should I go about this one?


  2. jcsd
  3. May 30, 2005 #2

    Doc Al

    User Avatar

    Staff: Mentor

    What you are trying to do for part a is actually the solution to part b! (Except for the sign.) In solving part a, you are making the same kind of error as was made in this post: https://www.physicsforums.com/showthread.php?t=77424 (See my comments there.)
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