physics question

Question

If an object is in equilibrium, which of the following is necessarily true?

A. The object is not rotating.

B. The object's center of mass is not moving.

C. The object's center of mass is not moving nor is it rotating.

D. None of the above are necessarily true.

Answer 1

Hi Zachariah W.,

It's D. An object can be happily moving linearly or rotating, without external forces acting (the definition of "in equilibrium" for a physics class!). That's just a matter of choosing a fram of reference, after all. But changing momentum, either linearly or angularly, that's a sign of an external force acting (or some other non-equiibrium process).

Inertia, now, that's an interesting concept. A commonly expressed thought is that inertia is conferred by distant mass in the universe, since if there was NO other mass in the universe, it would be meaningless to consider the inertia (or other properties, for that matter) of an object. What do you think of that argument, can you think of a good counterargument?

-- Cheers, --Mr. d.