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**1. Homework Statement**

A pith ball is charged by contact with a Van De Graaff machine, and is repelled away from the dome. It remains at its location. Using the chart given, what is the mass of the pith ball?

Distance between centre of generator and pith ball

0.25 m

0.30 m

0.35 m

0.40 m

0.50 m

0.60 m

0.70 m

angle of pith ball string

17.3º

15.8º

14.0º

10.8º

7.0º

4.9º

3.6º

electrostatic force (x10

^{-3}N)

5.5

5.0

4.4

3.3

2.2

1.5

1.1

**2. Homework Equations**

F

_{e}= kq

_{1}q

_{2}/r

^{2}

E = kq/r

^{2}

F

_{e}(is proportional to) 1/r

^{2}

**3. The Attempt at a Solution**

I made a graph using the proportionality equation, and calculated the slope to be 2.67. That means that kq

_{1}q

_{2}is 2.67, so I divided it by k, which is 8.99x10

^{9}, and got 2.97x10

^{-10}. So I know that the charges of the pith ball and generator multiply to equal that, but I don't know how to separate them or find the mass of the pith ball after that. I don't even know if what I've done will actually help me get an answer... none of the formulas I have relate mass to these numbers in any way unless I have an acceleration, and the pith ball is motionless so that's not possible.

If anybody could offer suggestions to help, I would appreciate it very much!