Here's another weekend physics question to contemplate - an excellent question for freshman phyics students to ponder:

Newton's law of inertia says that bodies in motion stay in motion, and those at rest stay at rest, unless a force acts upon them. Same applies to rotating bodies. You start a top spinning and it stays spinning unless a foce like friction interferes. Conversely, a top can't just start turn on its own starting from rest unless some outside force acts on it.

So here's the question: it is well known that if you suspend a cat upside down from a reasonable height and drop it, the cat is agile enough to land upright on its paws. So, how is the cat able to rotate in mid-air when there is no initial angular velocity imparted on it, and there is nothing for it to push against?:rolleyes:

Well, cats aren't uniform so their bodies have a moment of inertia around a point that's not directly at their center, this means that by pulling their legs in (i.e. increasing density) they can turn chemical (muscle) energy into rotational energy.

Not quite right - cats do indeeed pull their legs in, which can affect their rotational velocity(like the ice skater who pulls her arm in to spin faster). But if the cat starts falling without any rotational velocity to start with, what good does pulling in its legs do?

... do they spin their tails? :-)

I believe that they could not do it in an ideal frictionless situation. They use air friction to swing themselves around, much like if you sit in a cart, you can move forward through clever shifting of your weight to minimise friction when you're going forward, and maximising it when shifting your weight back to the starting position.