On Jan 27, 5:12 pm, Tina wrote:
Sorry. Rigid bodies do NOT rotate around their cg if an external force
is applied whose vector goes thru it.

Drop a yardstick, cg at the 18 inch mark, so that its zero inch edge
hits a table. The center of rotation as a reaction to that force is
the table edge.

You may write an equation that descibes rotation around its cg, and
another that describes translation, but a center of rotation, to many
who deal with such things, is that point on a rotating body whose
translational motion does not include rotation, the body appears to
rotate around it.

In the case I just described, such a point is at the end of the
yardstick.

You are obviously defining center of rotation differenrtly than I am,
but my American Institute of Physics Handbook on page 2-9 talks about
rotation "in which some axis or point remains fixed in space". That is
the center of rotation. In the several examples I've given that axis,
the center of rotation, is not at the center of gravity.

I am sure the math and classical physics folks use the same
definition. It's perfectly fine to talk abou other ways of describing
rotation, but engineers who think about it a little, even if they are
pilots, would tend, I expect, tend to agree with AIP handbook if they
are trying to communicate with other engineers.
.
As I claimed earlier, if allowed thusters on a rigid body, I can make
it rotate around ANY point. The table edge in my example could be
replace by such a thruster.

Now, if the forces are removed, you will get no argument from me that
rotation is about the CG. The forces are not removed in the OP's
question.

I think I see where Bertie's coming from. Rotation is
about the CG, while that CG is moving along some line due to external
forces.

Dan