electrostatics question...please help.

[prncess23]

please help with this problem...i have No clue how do it

Two pith balls of mass m are suspended by silk strings of length L. When they are charged they repel each other at an angle of 2 theta, since like charges repel. Using Coulomb's Law and Newton's Law, derive the following expression (http://img.photobucket.com/albums/v199/prncess20/physics.jpg) where k is the coulomb constant. The expression will be used to calculate the charge q on each ball.

[wysard]

What have you done in terms of attacking this problem?

We need to see your work and approach to offer input, we do not just solve the problem for you. Sorry.

[prncess23]

all i got was i wrote down the coulomb's law and newton's law (f=ma). after that...i really don't know what to do, to get that expression.

[hage567]

Have you drawn a diagram of the situation? This will help you relate distances, etc. How do these things fit into your equations?

Also, draw a free body diagram of all the forces acting on the ball. Sum them up in both directions according to Newton's second law.