# Silly physics...

Rob Park rbpark-NOSPAM at ualberta.ca
Wed Feb 12 13:09:14 PST 2003

```I was doing a physics lab the other day, and it was pretty messed up.

Basically, we had a pendulum with an electric charge, and we moved a
similar point charge near it, and we measured how the distance varied
with charge (sort of).

Anyway, things got interesting when we had to do the error calculations.
The equation for electric force is F=(kq1q2)/r^2, and to find the error
in F, you have to propagate the error from q1, q2, and r through that
equation with well defined and thoroughly disgusting propagation of
error calculations.

Suffice it to say, when you multiply by a constant, you also multiply
the error by that constant... and k just happens to be 8.99 * 10^9.

What I'm trying to say, is that the two electrically charged balls were
3 cm apart, give or take 400,000 km. Note that the moon is less than
400,000 km away from the Earth. So, we really have no way of being
certain that the little styrofoam balls weren't on the moon...

Ah, you just gotta love physics...

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Rob Park
http://www.ualberta.ca/~rbpark
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