You have the right idea about what the inverse-square law means, but...

The P=Q problem is easily solved since P and Q are points, but the
objects in question are not.  If they are spheres, then P and Q
represent the centres, so unless the spheres are infinitely small, then
P and Q can never be equal.

The problem with units arises because the force is *proportional* to,
not equal to, S / vlength(P-Q)^2.  A constant of proportionality is
needed, which obviously depends on the units.  For example,
gravitational force is F = (G * m(1) * m(2)) / r^2.  Here r is the
distance between the two masses, and m(1) and m(2) are the masses of
each object.  G is the constant of proportionality, equal to 6.67E-11
when the masses are in kilos and the distance is in metres.

PS magnets are nasty to work out, as they must have both poles, and
therefore a complicated field.  Use a charged ball to think about
instead - it only has one polarity to worry about!



"Andrew Coppin" <orp### [at] btinternetcom> wrote in message
news:3d95a3fe@news.povray.org...
> I'm trying to get my Chaos Pendulum scene to work, but it ain't
happening. I
> can't get the math right!
>
> Apparently, both gravity and magnetism work using the "inverse square
law of
> attraction". I don't actually *know* what that means, but I've given
it my
> best guess...
>
> Forget gravity, let's talk magnetism. Suppose I have a magnetic ball
at
> point P, and a magnet of strength S located at point Q (and fixed in
place).
> Now, I recon the force experienced by the ball is in the direction Q -
P,
> and it's strength is equal to the reciprocol of the square of the
distance
> from P to Q; i.e., force = S / vlength(P - Q) * vlength(P - Q).
>
> But hang on... wouldn't that mean that the way the force drops off as
we
> move away from Q is dependent on our units of measurement? Suppose P
is 1
> meter from Q. Then we have S / 1 * 1 = S. But if we write this as 100
> centimeters instead, we have S / 100 * 100 = S / 10000. Um... a little
> confused here!
>
> Wouldn't that also mean that if P = Q, then the force of the magnet is
> infinite? (Or more importantly, if P *almost* equals Q, the force
would be
> astronomically large.) Now I'm *really* confused!
>
> By the way... is the force excerted by a *real* magnet dependent on
the mass
> of the magnetic ball?
>
> OK, my head *really* hurts now...
>
> Help!
> Andrew.
>
>
>

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