Silly physics...

torsten torsten at
Wed Feb 12 13:40:41 PST 2003

On Wed the 12 Feb 2003 at 14 hours EST
RP wrote...
>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's your error on k?  Since you are calculating error on F, and
if k is assumed to be a constant, the the sum of errors won't include
the error on k.

You'll then find out, your balls are where you want them to be.


>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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