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Homework Help: Coulumb's Law Question

  1. Oct 6, 2006 #1
    Question:
    Two pith balls are free to float in space. If only gravitational and electrical forces are acting, and the balls have identical masses and charges (q = 4e), find the mass of the balls if they are moving at constant velocity.

    I'm pretty sure that there is something I'm missing because when I try to solve for the mass, there are just too many variables to work with. Here's my work:

    http://img291.imageshack.us/img291/2433/unsolvablemg8.png [Broken]

    Known values:
    k = 9.0 x 10^9 Nm²/C²
    G = 6.67 x 10^-11 N /m²kg²
    q = 4(1.60 x 10^-19 C)

    Otherwise, there are 3 variables missing if we solve for m ...
     
    Last edited by a moderator: May 2, 2017
  2. jcsd
  3. Oct 6, 2006 #2

    Doc Al

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    Staff: Mentor

    You set the distance equal... that's not very meaningful! Instead, what must be true about the forces if the balls move at constant velocity?
     
  4. Oct 6, 2006 #3
    Ohh! Constant velocity indicate that Fnet = 0. But I don't understand how that applies here. Surely, Fg and Fe cannot equal to 0 o_O
     
  5. Oct 6, 2006 #4

    Doc Al

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    Staff: Mentor

    They don't have to equal zero, but they have to add to zero. The net force on each ball must be zero.
     
  6. Oct 6, 2006 #5
    I can't believe I didn't realize this. Nonetheless, thank you very much :)
     
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