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Homework Help: What is the net charge of the ball?

  1. Mar 2, 2009 #1
    1.A small 3.94 g plastic ball is suspended by a
    27.9 cm long string in a uniform electric field
    of 3400 N/C.
    The acceleration of gravity is 9.8 m/s2 , and
    the Coulomb constant is 8.99×109 N · m2/C2.
    If the ball is in equilibrium when the string
    makes a 43.2◦ angle with the vertical as in-
    dicated, what is the net charge on the ball?
    Answer in units of μC.

    2. E=F/q

    3. Well I'm kind of lost. When they ask for the net charge, I just plugged the F, found using F=mg, and electric field into the equation E=F/q. And then I used that q and plugged it into the Fe=kq/r^2 equation, and found q. I thought that was the answer, but apparently not. I don't understand how to do the problem at ALL. please help. thanks in advance.
  2. jcsd
  3. Mar 2, 2009 #2


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    Homework Helper

    Welcome to PF.

    Maybe make a drawing of the charged object and put all your forces acting on it in?
  4. Mar 2, 2009 #3
    I tried that. but after putting the forces in, I dunt know where to go from there.
  5. Mar 2, 2009 #4


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    Homework Helper

    Yeah ... but.

    How might you explain the ball being at the angle described?

    Isn't there an m*g force down and a E*q force horizontally?

    Might that mean then that the vertical force is related to the horizontal force by some trig function of θ ?
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