Two small balls each of mass 15g are fastened to the ends of silk threads 1m long each and hung from a common point. They attain equilibrium at a distance of 20cm from each other. If one ball has a charge of -0.6microC, what is the charge of the other ball?
Both threads hang at an angle sin^-1 10/100 = 5.74 degrees from vertical.
The ratio of the vertical weight force to the horizontal repulsive Coulomb force is
tan5.74
= [k*Q1*Q2/(0.20)^2]/(M*g)
= 0.1005
k is the Coulomb constant. You may need to look this up.
Q1 = -0.6*10^-6 C
M = 0.015 kg
Solve for Q2. It will also be a negative charge.