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Finding the charge on two balls in equilibrium

  1. May 16, 2014 #1
    Two balls, each of mass 0.50g are each suspendedd from an insulating thread 1.0m long. The upper ends of the threads are attached to the same point. When the balls are equally charged they repel each other and are in equilibrium when they are 30cm apart.

    Find the charge on the two balls.

    F = KQq/r^2 (where K is equal to 9x10^9)

    I really appreciate any help on this one. As I can't seem to find the answer. I know triganometry is involved and I know you need to find TcosΘ and TsinΘ and divide them but after that I'm completely stuck.
    Last edited: May 16, 2014
  2. jcsd
  3. May 16, 2014 #2


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    The first step in such problems is always to draw a diagram of all the forces acting on the bodies. Then sum up the forces on each body.

    You also haven't mentioned what you are looking for.
  4. May 16, 2014 #3
    Oh, my apologies. I'm looking for the charge of the two balls.

    I've already drawn up a vector triangle.
  5. May 16, 2014 #4
    Start by drawing a free body diagram. Make sure you include all the relevant forces. Write the equilibrium equations.
  6. May 16, 2014 #5


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    I don't know exactly what you mean by a "vector triangle". You need to draw all the forces acting on the balls, regardless of whether they form triangles or not. You already mentioned the tension of the string, but there are other forces present. After you draw all the forces, break them into vertical and horizontal components.

    You did actually mention what you were looking for; I just didn't notice it.
  7. May 16, 2014 #6


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    Koan may be referring to a common method of dealing with statics problems which involves drawing a closed polygon with sides oriented as per the forces and lengths in proportion to the magnitudes. So for three forces you get a triangle.
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