[ODE] Subtractive Geometry

[Russ Smith]

> This brings to mind a question I've had for a while: do inertia
> tensors subtract?  I figured out a way to add them long ago in a
> galaxy far far away, but my patience ran out before I figured out how
> to subtract them.

interesting question. i think it depends on exactly what you're
subtracting. an inertia tensor has to be "positive definite", which
means that it represents a real mass. if you take the negative of say a
box inertia tensor that that would be bad. but it would be okay to take
a big box and subtract a small box from it's center. it might be bad
some times to subtract B from A if B covered part of the space that A
did not - but i don't think all such cases will be bad.

russ.

-- 
Russell Smith
http://www.q12.org