Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Instead of a flat disc one would expect the centrifugal forces to push it in..

The body is not in a container but suspended in air spinning horizontally (i.e. left to right or vice-versa) - how to do this could be challenging - but that is not the issue.

Answer: It is not prolate:

share|cite|improve this question
This question is too imprecise to be answered; how spinning? in what container? How is "oval shaped (vertically)"? – mbq Mar 10 '11 at 15:24
No ;-) How suspended? If in no-gravity situation, this is mostly a tradeoff between c-f making it a disc and surface tension making it a sphere, but the result is oblate not prolate. – mbq Mar 10 '11 at 15:43
I "read" that this is about a liquid at zero gravity, but of course the rotation of such a "drop" would not lead to a prolate ellipsoid, but more approach a disk (As I understand "prolate") – Georg Mar 10 '11 at 15:59
I would have thought so too but I heard otherwise. (one of us must be mixed up, I take prolate to mean like rugby ball,, and oblate to be like a disc..) – markmnl Mar 10 '11 at 16:02
A good general rule: Before asking "why," ask "whether." I'm not convinced that the phenomenon for which you're seeking an explanation actually occurs. Can you give a more specific example, or some sort of evidence that this occurs? – Ted Bunn Mar 10 '11 at 20:35
up vote 0 down vote accepted

It isn't prolate. It's oblate. Centrifugal forces push stuff out from the center. Nearby surface elements of the liquid are tugged along by surface tension, flattening out the original shape into a disk.

If something were to take on a prolate shape, there would need to be forces along the axis of rotation, which don't exist for most fluids I can think of, but if you provide a counterexample we could probably explain it. Beyond that, I think you're just confused.

share|cite|improve this answer
Did You read the comments to this question? "Everthing was said already, but not by everybody" – Georg Mar 10 '11 at 19:18
@Georg I don't know what you're so angry about. The original question was "Why does <thing that doesn't exist> exist?" and the only correct answer is, of course, "It doesn't." Even if this was discussed in the comments, stack exchange doesn't work off of comments, it works off of answers. – spencer nelson Mar 11 '11 at 17:00
If You read the whole thread (including times of answers and comments) You might learn why. – Georg Mar 12 '11 at 17:55

I don't know about liquids 'suspended in air', but a self-gravitating rotating fluid may take many shapes, not only oblate ones.

Jacobi has shown that at high enough spin rates the familiar oblate shape becomes unstable and turns into a triaxial ellipsoid. At even higher spins another instability appears (this one was described by Poincaré) and the system becomes pear-shaped.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.