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Homework Help: Two Electric Balls Hanging

  1. Sep 15, 2009 #1
    1. The problem statement, all variables and given/known data
    Assume each ball is loosing charge at a rate of 1.20nC/s, as what instantaneous relative speed do the balls approach each other initially.
    m=11.2g L=122cm x=4.70cm

    http://img40.imageshack.us/img40/9708/electballs.jpg [Broken]

    2. Relevant equations

    I figured since the value given is a rate it would be the dq=-1.20nC/s, but I am not entirely sure what exactly this rate is referring to, I am assuming it would be derivative to charge q. But the units are C/s. I am a bit lost now, because I don't know where to start, if I use Coloumbs law with dF and find force and divide it by mass that is given then I can maybe find acceleration and continue from there, but somehow I doubt it.

    I know that v=dx/dt, maybe I have to find a way to find these equations from the rate given.

    Thank You
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Sep 15, 2009 #2
    Ok, I have missed one huge glaring detail, that x is defined as x=[q^2L/2(pi)mg)]^1/3, I guess taking the derivative of this will result in the answer I am still trying to work it out, but it is not a simple derivative.

    If someone could please help a little. That would be great.

    Thank You.
  4. Sep 15, 2009 #3


    User Avatar

    Staff: Mentor

    What are the forces on the balls? Gravity and electrostatic forces, and the string tension, right? Draw a free body diagram, sum the forces, and use that to calculate the motions. The fact that the charge is leaking away at some rate does complicate the electrostatic force components, but should still be manageable.
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