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Homework Help: Dynamics electricity problem

  1. Feb 20, 2008 #1
    In an electricity experiment, a 1.10 g plastic ball is suspended on a 59.0 cm long string and given an electric charge. A charged rod brought near the ball exerts a horizontal electrical force F on it, causing the ball to swing out to a 19 degree angle and remain there.

    What is the magnitude of F?
    What is the tension in the string?

    Ok, I set up my free body diagram and set the forces along the y-axis = 0.
    I came up the the equation, .1078cos(19) + F -mg=0 , which does not give me the right magnitude for F, what is wrong with my equation? The length of the string is only needed for the tension part of problem correct? But I do not know how to set that up either.
  2. jcsd
  3. Feb 20, 2008 #2
    Well vertically you have:
    mg = Tcos19

    And horizontally:
    F = Tsin19

    So I'd find the tension first, then use that to find the electrostatic force on the ball.
  4. Feb 20, 2008 #3
    this isnt really what id call my good point in physics but i would say that you mite need to use the length of the string for more then just the tension.. set up ur thingo with various lengths of cord and check the differences.... this probably didnt help u much but as i said,,, not my good point
  5. Feb 20, 2008 #4
    Ok so I came up with T=.0114 and F=.0037N, which is correct. Thank you!
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