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Homework Help: Finding Tension in String Connected to Electrically Repelled Sphere

  1. Jan 15, 2013 #1
    1. The problem statement, all variables and given/known data

    Two uniformly charged spheres are suspended by strings of length L from vertically adjustable supports. The spheres are in static equilibrium.

    The angles with respect to the vertical are Q=14.9°, and T=20.7°.

    The tension in the string supporting sphere Z (Z with respect to angle T, W is the other sphere and hangs with respect to angle Q) is 2.41E-5 N. Calculate the tension in the other string.

    2. Relevant equations

    Fe = k (q1 * q2 / (r) ^ 2)

    Fg = mg

    3. The attempt at a solution

    Alright, here is a brief overview of what I have done. I first recognized that the system was in static equilibrium. Thus, for each sphere I wrote equations representing the force components, and set each equation to zero. Each sphere operates under the same conditions.

    Sphere Z lies to the left (T is force due to tension, Fe is force due to Electric repulsion)

    Tx = Tcos∅
    Ty = Tsin∅
    T = 2.41E-5 N

    0 = Tx - Fe
    0 = Ty - mg

    Sphere W lies to the right:

    Tx = Tcostheta
    Ty = Tsintheta

    0 = -Tx + Fe
    0 = Ty - mg

    So, I recognize that in each case, the x-component of the tension in each string is equivalent. This makes sense because the forces on each sphere with respect to the x-axis are due to the electric force. This force has an equal and opposite effect. I cannot figure out how to find the total tension in the second string, even though I know the x component etc. Any help in progressing from this point would be greatly appreciated.
  2. jcsd
  3. Jan 15, 2013 #2
    The question states that the spheres are on vertically adjustable supports. Are we allowed to assume that the spheres are at the same height, because this would simplify the question to equating horizontal tension components.
  4. Jan 15, 2013 #3
    Yes, absolutely. There is a diagram I can include if that is needed. The horizontal components are equivalent. However, the masses are not given, so I haven't a clue how to find the y component of the tension in the second string.
  5. Jan 15, 2013 #4
    But you have its x component Tx and an angle ∅ and Tx=Tcos∅
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