To celebrate Linda's changing the blurb of this forum ( ), I want to put forward a question that has been bothering me for ages.

A centre of mass of an object is "the point where the whole mass of the object is said to be concentrated". It is at this point where, if the object is placed on a rod or something similar, it doesn't tilt either way.

If this is a stick:
__________________________________________
|_____________________M___________________|

the centre of mass would be located around the point 'M'.

In school, however, I've been taught that the centre of mass of a body is always located within the body itself. If this is so, then where is the centre of mass of a ring?

If I recall correctly, the "center of mass" (or "center of gravity") does NOT have to be actually within the object. It's strictly a mathematical fiction, anyway, used to simplify calculations; the object can be treated AS IF the mass were located in the one point in certain ways. Hence, the center of mass for the ring is, indeed, in the middle of the hole.

That's what my mother told me as well, that it doesn't have to be within the mass of the object. She says that you can take a 'centre of mass' for even a 'field' of particles, and it doesn't have to be within the body.

I guess my teacher was wrong then!

Thanks for that, VickiSilver!