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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?

    http://i694.photobucket.com/albums/vv301/redraven45/physics1_zps6759b55e.png



    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
    =0.882N
    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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