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Problem With Repulsive Charges

  1. Jan 28, 2012 #1
    1. The problem statement, all variables and given/known data

    Two 3.0g spheres on 1.0m-long threads repel each other after being equally charged, as shown in the figure.
    jfk.Figure.20.P64.jpg

    2. Relevant equations

    F = k*q1*q2/r2

    3. The attempt at a solution

    sin 70° = Ty/T
    T = (m*g)/sin 70°
    T = 0.003*9.81/sin 70°
    T = 0.03132 N

    Tx = T*cos 70°
    Tx = (0.03132)*cos 70°
    Tx = 0.01071 N

    Since both objects experience this force, the force is doubled at 0.02142 N.

    1/2*r = sin 20°
    r = 0.6840 m

    T2x = k*q2/r2
    q = (T2x*r2/k)1/2
    q = [(0.02142)(0.6840)2/(8.99*109)]1/2
    q = 1056 nC = 1.056* 10-6 C

    Where am I going wrong?
     
  2. jcsd
  3. Jan 28, 2012 #2

    PeterO

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    Homework Helper

    I suggest that the line in red is where you went wrong.

    during a "tug-of-war", when two teams pull on each end of a sturdy rope, each team pulls with the same force [Newton's third law will show why the forces are equal] but the tension in the rope is just that same value. ie. if each team pulls with 10 000 N, the tension in the rope is 10 000 N [not 20 000 N]
     
  4. Jan 28, 2012 #3

    BruceW

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    I agree
     
  5. Jan 28, 2012 #4
    sin 70° = Ty/T
    T = (m*g)/sin 70°
    T = 0.003*9.81/sin 70°
    T = 0.03132 N

    Tx = T*cos 70°
    Tx = (0.03132)*cos 70°
    Tx = 0.01071 N

    1/2*r = sin 20°
    r = 0.6840 m

    Tx = k*q2/r2
    q = (Tx*r2/k)1/2
    q = [(0.01071)(0.6840)2/(8.99*109)]1/2
    q = 747 nC = 7.47* 10-7 C

    How about now?
     
  6. Jan 28, 2012 #5

    BruceW

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    Homework Helper

    looks good to me.
     
  7. Jan 28, 2012 #6
    Got it, thanks guys. :)

    (747 nC is correct).
     
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