Re: Axis of rotation

Uncle Al wrote:

matt271829-news@xxxxxxxxxxx wrote:

OK then, I'm confused. At
http://www.uwgb.edu/dutchs/PLATETEC/RotationQk2004.HTM, which seems a
reputable site, it says, of the earth's axis wobble: "Objects that are
not perfectly spherical do not rotate around a single axis. That's just
the laws of physics."

Is this really true? I always thought that any rotating rigid object,
in the absence of any external forces, will (and indeed *must*) rotate
about a single, fixed axis. Who's right?

The Earth's mass sloshes about (atmosphere, seas, molten innards,
earthquakes, vulcanism, anthropogenic construction) so the position of
its spin axis relative to its surface geography physically moves. The
position of the spin axis relative to the fixed stars does a pretty
good boogaloo from several components.

Relevant to the Earth, but not, of course, to the question of why a
rigid object may wobble.

A rigid rotating body has but one spin axis.

At one moment... it may change in time.

If its three moments of inertia are all different it must tumble vs. the fixed stars.

For suitable values of "must" (not rotating about one of the principal
axes, subject to stability).

Note to O.P.: You ask very good questions! I can answer some but not
all now. I shall endeavor to bring myself up to speed, by which time,
of course, you may have lost interest or someone else will have given a
more complete answer.

Matrix algebra and calculus will suffice. What's your level?

.

Relevant Pages

  • Re: Axis of rotation
    ... reputable site, it says, of the earth's axis wobble: ... not perfectly spherical do not rotate around a single axis. ... Matrix algebra and calculus will suffice. ...
    (sci.physics)
  • Re: Axis of rotation
    ... reputable site, it says, of the earth's axis wobble: ... not perfectly spherical do not rotate around a single axis. ... It gets a little more complicated than that- some axes are more stable ...
    (sci.physics)
  • Re: Axis of rotation
    ... reputable site, it says, of the earth's axis wobble: ... not perfectly spherical do not rotate around a single axis. ... It gets a little more complicated than that- some axes are more stable ...
    (sci.physics)
  • Re: Axis of rotation
    ... reputable site, it says, of the earth's axis wobble: ... not perfectly spherical do not rotate around a single axis. ... If the object is rotating about one of the "principal axes of rotation" then the object can rotate about that axis forever. ...
    (sci.physics)
  • Re: Axis of rotation
    ... not perfectly spherical do not rotate around a single axis. ... about a single, fixed axis. ... J, the angular momentum, remains fixed in spatial orientation in the ...
    (sci.physics)