# Bottle spin

why do bottles or any other things thrown up with a twist always spin about their centre of gravity? Also if all forces pass through centre of gravity of a rocket in space, it will not rotate , even though there is a net moment about any other point except the cg. why

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pervect
Staff Emeritus
Any spinning object has to have some axis around which it spins, just by the nature of what it means to spin.

The axis of spin has to pass through the center of gravity in order to satisfy neewton's laws. Imagine twirling a big long bat around the endpoint, rather than the middle. You can make it spin like that, but you have to apply a force to do it.

A more detailed explanation is possible, but requires some math. You can usually see a derivation in terms of the sum of the forces in most books on dynamics.

consult a good physics boom on rotational/transational momentum.

Gonzolo
aerobilal said:
why do bottles or any other things thrown up with a twist always spin about their centre of gravity? Also if all forces pass through centre of gravity of a rocket in space, it will not rotate , even though there is a net moment about any other point except the cg. why
An objects spins around its center of mass, not it's center of gravity. These are distinct points that often coincide, but not necessarily. The center of mass is the point that is fix, no matter what the axis of rotation is.

The center of gravity is the point which would have the same trajectory in a gravity field as a the entire body.

They differ when gravity is not constant throughout the object (such as a mountain or a very very tall post).