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Homework Help: Tension on a string and charge - need help

  1. Jul 23, 2013 #1
    1. The problem statement, all variables and given/known dataObject A has mass 90.0 g and hangs from an insulated thread. When object B, which has a charge of +130 nC, is held nearby, A is attracted to it. In equilibrium, A hangs at an angle θ = 7.20° with respect to the vertical and is 5.00 cm to the left of B.
    (a) What is the charge on A?
    (b) What is the tension in the thread?


    2. Relevant equations
    F = ma
    F = kq1q2/r^2

    3. The attempt at a solution
    Im pretty clueless on this one. I know it's some type of equilibrium question, and it looks like the forces on the x direction cancel out. To find the charge on A, I need the force between the two charges but I don't have that and I'm not sure if I'm doing this right.

    The only force on the string in the y direction is from gravity so

    Fy = 0.09*9.8
    The only force pulling the string in the x direction is the force between charge A and charge B
    tantheta = Fx/Fy
    Fx = tantheta*Fy
    = tan7.2*0.882
    = 1.15N

    This equals to the force between the two charges, so

    1.15 = k(qA)(qB)/(r^2)
    (qA) = 1.15 *0.05^2/ (8.85e-12)(130e-9)
    (qA) = 2.49e15

    this value doesn't make much sense. not sure what to do.
    Last edited by a moderator: Jul 24, 2013
  2. jcsd
  3. Jul 23, 2013 #2
    The angle is in degrees not in radians:

    ##\tan 7.2^o=0.126\Rightarrow F_x=0.111\,N##

    and ##k=9\cdot 10^9\,N\cdot m^2/C^2##
  4. Jul 23, 2013 #3
    thanks for the correction, my final answer for the charge is now 237 nC. That seems a lot better

    Also, the force of tension I got was sqrt((0.0882^2)+(0.111^2))= 0.122N

    Can you confirm that these are correct?
  5. Jul 23, 2013 #4
    The charge is ok, but ##F_y=0.882\,N## not ##0.0882##
  6. Jul 23, 2013 #5
    Thanks man, just got to pay more attention to these clumsy errors I guess.
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