# Problem - Coulomb's Law

1. Oct 12, 2004

### anathema

I am having a problem with the following question:

Two identically charged ping pong balls each having a mass of 4.00 g are hung from fine threads, 0.75 m long. What is the charge on each ball so that the angle between the threads is 35 degrees?

I don't know how to draw a free body diagram for this problem in such a way that the forces cancel out. Do I just take the vertical component of the gravity and have it cancel with the strictly horizontal electric force, and assume that the vertical component of the gravity is canceled out by the tension of the string? Do I find a way to have both forces acting perpendicularily to the string (and thus tangentially to an arc of motion)?

2. Oct 12, 2004

### maverick280857

Hey you seem to have figured out the problem correctly. The three forces (as you correctly reasoned) are tension, gravity (weight) and the electrostatic force. Gravity always acts vertically downwards, electrostatic force acts horizontally and tension acts at an angle (making 35 degrees with the vertical). So tension has two components...one that balances weight and the other that balances electrostatic force. Now the electrostatic force is that of repulsion not attraction so the horizontal component of tension and the electrostatic force are antiparallel. Draw a diagram now to convince yourself that this is so.

Hope that helps...

Cheers
Vivek