Finding Charge of Two Charged Spheres

[roam]

Homework Statement

Two hard rubber spheres, each of mass m = 15.1 g, are rubbed with fur on a dry day and are then suspended with two insulating strings of length L = 4.90 cm whose support points are a distance d = 2.97 cm from each other as shown in the figure below. During the rubbing process, one sphere receives exactly twice the charge of the other. They are observed to hang at equilibrium, each at an angle of θ = 10.3° with the vertical. Find the amount of charge on each sphere.

[PLAIN]http://img269.imageshack.us/img269/4021/44539563.jpg

Homework Equations

The Attempt at a Solution

I'm not sure how to approach this problem. If the charged particles are in a uniform Electric Field then

$\vec{F} = q \vec{E} = m \vec{a}$

$\vec{a} = \frac{q \vec{E}}{m}$

The problem says "hang at equilibrium", if this means they're not moving then the acceleration of the particles is 0. If it means electrostatic equiblirium, then the we know the electric field is 0 everywhere. So what could we do? How should one appraoch this problem?

 

[lackos]

dont forget about gravity

it is both the force of gravity and electrostatics that allows them to keep this equilibrium.

lackos

 

[roam]

dont forget about gravity

it is both the force of gravity and electrostatics that allows them to keep this equilibrium.

lackos

I don't understand, how could I use this to find the charge on each sphere?