1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Coulombs law + trig

  1. Sep 9, 2010 #1
    Sorry if I'm doing this wrong... first time poster.

    1. The problem statement, all variables and given/known data


    The diagram gives the relevant info. All three q's are equal point charges of m=0.10 kg. I'm trying to find the value of q.

    2. Relevant equations

    Coulombs law and basic trig.

    3. The attempt at a solution

    Considering either of the outer point charges, I have thus figured out that F_x=Tsinθ and mg=Tcosθ. Dividing those gives F=mgTanθ=0.98N

    I can't figure out what to do at all for the second part where I use Coulombs law to combine the the forces of both charges on the most right one. My prof gave the following HW explanation, but I don't understand it:

    [tex]\frac{Kq^2}{(Lsin\Theta)^2}[/tex] -- [tex]\frac{kq^2}{(2Lsin\Theta)^2}[/tex] = [tex]\frac{5kq^2}{4L^2(sin^2)\Theta}[/tex]

    I can rearrange and solve from there... I understand where [tex]\frac{Kq^2}{(Lsin\Theta)^2}[/tex] -- [tex]\frac{kq^2}{(2Lsin\Theta)^2}[/tex] comes from but how in the world does that turn into [tex]\frac{5kq^2}{4L^2(sin^2)\Theta}[/tex]

    Ugh it turned my thetas into 952;... I hope you still understand it.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Sep 10, 2010 #2


    User Avatar
    Homework Helper

    Hi prof chaos, welcome to PF.

    It is simple addition.

    1 + 1/4 = 5/4.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook