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Homework Help: Coulumb's Law and force between charge

  1. Jan 23, 2009 #1
    1. The problem statement, all variables and given/known data

    A ball, which we can treat as a point charge, has a charge of +Q and a weight of 9.00 N. This ball is 50 cm away from a ball of charge -10Q, which is fixed in position. The +Q ball is 30 cm vertically below, and 40 cm horizontally, from the other ball, and the +Q ball is held in equilibrium at that position by a string which happens to be perfectly horizontal

    Image: http://www.webassign.net/userimages/81225?db=v4net

    1) What is the magnitude of the electrostatic force that the ball with the charge of -10Q exerts on the ball with the charge of +Q?

    2) What is the magnitude of the tension in the string?

    3) What is the value of Q?

    2. Relevant equations

    Fn = mgcos0
    Fe = k(q1)(q2)/r^2
    Fw = mg

    3. The attempt at a solution

    I know this problem needs to be broken down into components, so I started off trying to figure out how to break up the Forces by drawing a free body diagram. On the -10Q ball there is Fn pointing upwards and mg directed downwards correct? To get the diagonal force I have to use mgcos0 which becomes mg(30/50)...I do not know if this is correct because I am trying to sum up all the forces in the x direction and all the forces in the y direction. As for finding the tension, I know it involves the same kind of process but I am confused on where to start since I think I need the answer to 1). with that answer I will be able to find answer 3 using coulomb's law.

    Thanks for all the help!
  2. jcsd
  3. Jan 23, 2009 #2
    To answer part 1, why don't you just directly solve your second equation? You have q1, q2, and you can easily find r.
  4. Jan 23, 2009 #3
    i don't know the value of q though.
  5. Jan 23, 2009 #4
    Draw a free body diagram for +Q. Three forces are acting on +Q: the electrostatic force by -10Q, the force of gravity, and the tension in the string. Resolve the electrostatic force, Fe, into vertical and horizontal components. The vertical component of this force must be equal and opposite to the weight, and the horizontal component must be equal and opposite to the string tension. With these equations, Fe and T can be found.
  6. Jan 24, 2009 #5
    chrisk thanks for the help, I figured it out in like 5 minutes..I cant believe didnt realize it before.
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