# A question on Electrostatics

1. Feb 24, 2005

### Neerolyte

two small spheres, each with 0.120g, have equal charges placed on them. Each sphere is suspended from the same point by its own silk fibre which are 75.0cm long. The repulsive force between them keep their centres 10.0cm apart. What is the charge on each sphere?

Last edited: Feb 24, 2005
2. Feb 24, 2005

### Staff: Mentor

It's a static equilibrium problem. Start by identifying all the forces acting on each sphere.

3. Feb 24, 2005

### Neerolyte

hm...this is what i think of it

Since there is strings involved, there must be tension. And since there are 2 strings connected to 1 point holding the 2 spheres. There must be T1 and T2. T1 and T2 must have components. We knew the mass of the two spheres so we know what Fg is, which equals to T1y and T2y.

Here is thing i don't know. Is T1x the repulsive force? IF it is, why does the data contain the string's length...and IF it is NOT, then i'm not sure how to do it...

yes i did a FBD, and identified all the forces. Fg, T, and repulsive force, that's it..

4. Feb 25, 2005

### Staff: Mentor

Right. But the symmetry of the problem tells you that T = T1 = T2; you only need to deal with one of the two masses. And yes, the tension in the string exerts a force on the mass with components Tx and Ty. For equilibrium, Ty = mg.

The only forces acting horizontally are Tx and the repulsive force (F), so F = Tx. The length of the string and the distance between the masses will allow you to calculate the x and y components of the tension force. (Hint: One will be Tcos(theta); the other will be Tsin(theta).)

Next step: Rewrite the two force equations and solve for the repulsive force F.

5. Feb 25, 2005

### Neerolyte

yep got it ^_^

thank you very much