Two charged spheres suspended from a point repel 24cm apart

[pugfug90]

Homework Statement

2 small spheres each having a mass of 0.1g are suspended from the same point on silk threads 20cm long. When given equal charges, they repel, resting 24cm apart. Calculate charge of each.

Homework Equations

F(w)=mg(a)
F=kQ(1)Q(2)/d^2
k=8.987x10^9 N*m^2 / c^2

The Attempt at a Solution

I converted grams to kilograms.. 1 x 10^-4 kg
kilograms to Newtons/weight..9.8*10^-4N

blah blah..
F=kQ(1)Q(2)/d^2
(7.35*10^-4N)(.24^2)(8.987*10^9 N*m^2/c^2)=Q^2
Q=6.86*10^-8C

http://i80.photobucket.com/albums/j199/pugfug90/physucks.gif
Here's some work and a diagram. I am 99% sure that the answer is 6.9*10^-8 coulombs. However! I don't understand.. If I used trig to find half of the total force (in the triangle diagram).. why do I use the full distance?

 

[hage567]

If I understand what you're asking: You aren't finding just half the force. Coulomb's law obeys Newton's third law, so the force exerted on Q1 by Q2 has to have the same magnitude as the force of Q2 on Q1, it is just in the opposite direction.

 

[pugfug90]

AHHHHHHH

But mah physical teacher drew this line between both balls (for notes) and labeled it force.. Hage, that kind of makes sense. Thanks for help. So force isn't really "half", therefore just because I'm using "half force" doesn't mean I use half distance..?

Anyone want to do the problem, starting with..

"2 small spheres each having a mass of 0.1g are suspended from the same point on silk threads 20cm long. When given equal charges, they repel, resting 24cm apart. Calculate charge of each."

Also, any online links with similar problems?
----

http://i80.photobucket.com/albums/j199/pugfug90/physicismean.gif
Because of that image, and a nice guy that kind of explained it.. I made this graphic and I hope it shows that I understand:) What do you guys think of it?

 

[pugfug90]

Hey hey can anyone confirm my work of art?

 

[pugfug90]

heyheyheyheyhey

 

[pugfug90]

KK, asked my teacher, he agrees with that pic, just wouldn't tell me upright..
https://www.physicsforums.com/showthread.php?t=156815
PS, I need help on this one:)