Silly physics...

Jochen Schröder jschrod at
Wed Feb 12 15:33:40 PST 2003

Rob Park 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 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...
Sorry but no! You have to multiply by k to get the error in F. Now if 
you want to calculate the error in r=distance between balls, you have to 
divide by k again. So you end up with a much lower error.
[X] Nail here for new monitor

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