Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: 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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook