Two cylindrical glass beads each of mass m = 22.0 mg are set on their flat ends on a horizontal insulating surface separated by a distance d = 1.07 cm. The coefficient of static friction between the beads and the surface is μs = 0.291. The beads are then given identical charges (magnitude and sign). What is the minimum charge needed to start the beads moving?

Question

anonymous · Answer

I solved for normal force by multiplying the mass in grams by g to get the normal force, and then multiplied by the coefficient of static friction to get the maximum force of static friction.  I set that value equal to the force of the charges.

So, mgu_s = kq^2/r^2.  I'm getting an answer of about 2.8 x 10^-8, but according to the assignment, the answer is a number times 10^-10.  I'm not sure where I'm getting the two magnitudes of difference off.

anonymous · Answer

I get about 8.94 x 10^-10 C.  I'm wondering if you forgot to convert to kilograms, not grams, if you are using MKS units throughout.

anonymous · Answer

Yep, you were right.  I actually was converting them but for some reason I read the mass as being in grams, not milligrams.