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Homework Help: Pendulum w/electrostatic force

  1. Mar 1, 2006 #1
    Two small metallic spheres, each of mass 0.28g are suspended as pendulums by light strings from a common point as shown. The spheres are given the same electric charge, and it is found that the two come to equilibrium when each string is at an angle of 5.1 degrees with the vertical. The Coulomb constant is 8.98755e9 and the acceleration of gravity is 9.81. If each string is 22.5cm long, find the magnitude of the charge on each sphere. Answer in units of nC.


    sin 5.1 = x/.225 x=.02m

    mg = (kq^2)/r^2

    (2.8e-4)(9.81) = ((8.98755e9)q^2)/(.04^2)

    q=2.2113e-8 C = 22.113 nC

    My answer is wrong and I don't understand why.
  2. jcsd
  3. Mar 1, 2006 #2
    Yeah,indeed they are, but not to worry. Look dude, in this case, what you need to realize is that there are multiple forces (in both the x and y direction), which are both electric and gravitational by cause. For both of the masses, the mg vectors point straight downwards, while the TENSION vectros lie along the string. AS for the electri force vectors, those point directly at or away from the particles (depending on the sign) and are along what would be an imaginary x axis. by equation all the forces vertically and horizontally (using newtons law) and seting the net forces equal to zero,you can find the charge of the two masses, which ill probably be equal given the equal masses and the symmetry of the problem. Take that into your efforts and show us how the calculations go. Good luck!
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