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Finding the angle from two charges repelling each other.

  1. May 8, 2009 #1
    1. The problem statement, all variables and given/known data
    The figure shows two 5.0 g spheres suspended from 1.0-m-long threads. The spheres repel each other after being charged to +91 nC. What is the angle θ ?


    2. Relevant equations

    I was thinking momentum and/or kinetic energy equations.
    k = 1/2 mv^2
    p = mv
    F = ma

    3. The attempt at a solution
    Answer is: 4.1 degrees
    I am not sure how to go about this problem other than from the equations I have already given.
     
  2. jcsd
  3. May 8, 2009 #2

    dx

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    Where's the figure?
     
  4. May 8, 2009 #3
    Sorry. I just attached it to a file here, so maybe you can see it now.
     

    Attached Files:

  5. May 8, 2009 #4

    dx

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    What forces are acting on the spheres? Which direction must the resultant force on each sphere point in equilibrium?
     
  6. May 8, 2009 #5
    Well, there is the electric force. F = qE = Kq_1q_2/r^2.
    In equilibrium, they must be pointing in opposite directions, right?
     
  7. May 8, 2009 #6

    dx

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    What about the tension force from the thread?
     
  8. May 8, 2009 #7
    So there is the electric force and the tension force. So Ftotal = T plus qE, right?
    I am unsure of how to calculate the tension with no velocity. So, still a little confused.
     
  9. May 8, 2009 #8

    dx

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    There's also the gravitational force, i.e. the weight. Draw a free body diagram with all of these forces with their proper directions. The tension points along the string inward. That means that the sum of the weight and the electrostatic force must point along the string but outward in equilibrium.
     
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