Two identical pith balls,each carrying a charge q,are suspended from a common point by two strings of equal length l1 &l2.find the mass of each ball if the angle between the strings is 2a in equilibrium.

Dear Nikhil

Let the lengths of the two strings be equal to 'L'
The distance between the two pith balls will be, r = 2Lsinθ
now, here
electrostatic force = gravitational force
kq2/r2 = mgtanθ
or
kq2 / (2Lsinθ)2 = mgtanθ
or
m = kq2 / gtanθ(2Lsinθ)2
thus,
mass of each ball will be
m =  kcosθq2 / 4gL2(sinθ)3
 
Regards
Arun (askIITians forum expert)