# How to find the mass of a pith ball using electric forces?

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1. Oct 12, 2016

### Shannon Smyth

1. The problem statement, all variables and given/known data
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. Relevant equations
Fe = kq1q2/r2

E = kq/r2

Fe (is proportional to) 1/r2

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 kq1q2 is 2.67, so I divided it by k, which is 8.99x109, 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!

2. Oct 12, 2016

### Simon Bridge

What other forces could be on the ball besides the electrostatic one?
Did you draw a free body diagram for the ball?