# Can Centripetal Force create Gravity?

1. Jul 1, 2009

### oragami

What i'm trying to get at is this, can a spinning framework of rings build enough centripetal force to keep a mass of liquid at the centre of the frame work? And if so what would be the relationship and conditions that would have to be met?
Really need help:|

2. Jul 1, 2009

### mathman

A spinning fluid would tend to go the outside, not toward the center. That is the principal of the centrifuge.

3. Jul 1, 2009

### oragami

well thank you mathman

4. Jul 1, 2009

### oragami

but i ment if the sphere was spinning and like a gas was in it, if most of the gas would travel towards the centre. Please advice

5. Jul 1, 2009

### math_04

Is that the whole notion behind artificial gravity in a spacecraft? A spinning spacecraft can reproduce the effects of gravity if I am not mistaken?

Last edited: Jul 1, 2009
6. Jul 2, 2009

Staff Emeritus
The gas will not go to the center. Why do you think it would?

7. Jul 2, 2009

### HallsofIvy

NO, it wouldn't. It would travel toward the circumference. That was mathman's point.

8. Jul 2, 2009

### HallsofIvy

People walking on the inside wall of the spinning space craft (heads pointed toward the axis of rotation) would feel a force pushing them against the space craft. Than can "simulate" gravity but will not be exactly the same. For example, in all but an immense space craft, coriolis forces would be evident.

9. Jul 3, 2009

### Chronos

A spinning fluid mass would resemble a galaxy in deep space. Gravity limits the effects of centripetal force.

10. Jul 3, 2009

### Wallace

I have no idea what you are trying to say? In a galaxy gravity is what provides the centripedal force that keeps it together??

11. Jul 3, 2009

### mathman

In a galaxy the centripetal force is gravity.

12. Jul 4, 2009

### Wallace

Yes, that's what I said? At least that was what I intended to say....

13. Jul 9, 2009

### JCOX

Yes...Theoreticaly this is possible. For simplisity sake take for example a ring and contain within it two elements... one a gas and other liquid. Let the gas be significantly "heavier" than the fluid. When spinning centripetal force would pull the heavy gas to the outside and leave the fluid (lightier) in the center...Is this even what you are asking???

Last edited: Jul 9, 2009
14. Jul 9, 2009

### JCOX

However, I don't know of any Gas heavier than liquid... so in accuality I dont think so

15. Jul 13, 2009

### raknath

So is the difference in behavior mainly due to the atomic freedom that exists in gasses

Does this amount to saying that randomly circulating gas molecules cannot be bound by the centripetal force?

How does gravity keep them together, is gravity stronger than centripetal force?

16. Jul 13, 2009

### JCOX

Can you please explain in more depth what you are saying? What centripetal force are you refering to??

17. Jul 13, 2009

### Rymer

Several levels of issues here:

1) One is called 'miscibility' see: http://en.wikipedia.org/wiki/Miscibility

2) Whether 'randomly circulating gas molecules' are 'bound' depends on their energy and strength of the forces involved.

3) In this particular case, the substances appear to be already contained -- according to the original question. So this should not be an issue.

4) Actually, in the case as presented -- a gas -- the presence of gravity or centripetal or any force is really unimportant. The gas is going to fill the volume.

5) A liquid would only maintain being a liquid in the presence of additional gas (of some kind) filling any extra volume (or vaporize to expand to fill the volume)

6) If the containing object (spaceship) is spinning, the denser material will settle against the outer 'walls'.

Last edited: Jul 13, 2009
18. Jul 14, 2009

### raknath

Consider a box with some gas and a lot of holes(very hypothetically)

Let us say i send the box into revolutions, now by what we have seen the gas would escape out of the holes, as the box rotates?

Now however if the gas is gravitationally bound like it is for say jupiter, the gas does not escape, i mean why is this any different? Does this mean that gravity influences gas molecules differently than it is affected by centripetal force?

19. Jul 14, 2009

### Janus

Staff Emeritus
A centripetal force is one that acts toward the center, so gravity is a centripetal force.

In your spinning box, the centripetal force is supplied by the walls of the box pushing inward on the gas preventing the molecules from following a straight line path as would be natural to them. When you put holes in the box, you have areas where there is no centripetal force to contain the gas and it escapes.

20. Jul 14, 2009

### D H

Staff Emeritus
Better: Gravity is a central force. Centripetal force is a kinematical rather than dynamical concept. This page, http://encyclopedia.stateuniversity.com/pages/3970/centripetal-force.html [Broken] explains central versus centripetal versus centrifugal forces in a nice, simple manner.

What centripetal force? You are acting as if centripetal force is something real in and of itself. It isn't. The real (i.e., physical) force in your rotating box is the electrostatic repulsion between the surface of the box and the gas molecules that hit it. The mass of the gas itself is insignificant. The real force in the case of Jupiter is the mass of all the stuff that comprises Jupiter. Compare this to the gas in a box situation. The gravitational force exerted by the gas on a gas particle near the surface of the box (or anywhere inside the box) is immeasurably small.

Last edited by a moderator: May 4, 2017
21. Jul 15, 2009

### raknath

Why so?

The holes are covered with air which in turn would be atttracted towards the center so there should be a force the air exerts towards the center, wont it?

However the reaction force of th gas still wins, how?

22. Jul 15, 2009

### raknath

No i am a little confused, Are you saying that the sum total of the force exerted by all the particles of the gas is too less to actually get affected by the centripetal force or is it that each molecule is actually never effect and it is the sum effect that we see manifesting. So every molecule is free to do as it pleases?

Last edited: Jul 15, 2009
23. Jul 15, 2009

### D H

Staff Emeritus
What force? Draw a free body diagram.

You are acting as if centripetal force is a real force. It isn't. Centripetal force is a kinematic force -- it is just a label we slap on the kinematics when the acceleration is, for whatever reason, observed to be directed toward some center.

Suppose this box (without holes) and the gas inside initially are non-rotating. Now start the box rotating. Aerodynamic drag will make the layer of the gas near the surface of the box more-or-less move with the box. This is called the "no-slip condition". A video: .

If you have a fan to keep you cool in the summer, look at the blades. They're probably a bit dirty. They are dirtiest exactly where you wouldn't expect it: Right where the air flow is greatest. That's the no-slip condition at work. Any dust that happens to hit the blade might well stick to the blade due to adhesion. It is adhesion between the air molecules and the fan blade that is responsible for this boundary layer effect. Because the boundary layer is pretty much at rest with respect to the blades, the motion of the blade through the air will not blow the dust off. Because the air flow constantly refreshes the boundary layer, there is an influx of new dust particles that can stick to the blade.

Back to the box that is just starting to rotate. Away from the gas/box boundary the gas is still not rotating. There is a wind inside the box. This wind transfers momentum from the boundary layer to the non-rotating gas. Eventually the entire contents of the box will be rotating (and the wind will vanish).

The only forces acting on a parcel of air in this box are gravity and the electrostatic force. The adhesion that is responsible for the no-slip condition is an electrostatic effect. The viscosity of the gas that creates the wind that in turn makes the entire box rotate is also an electrostatic effect.

Last edited by a moderator: Sep 25, 2014
24. Jul 20, 2009

### oragami

Ok i guess i was a little vague before. I was thinking of the magnetic levitation effect and was wondering if it was possible to achieve on an object at the centre of a series of rotating frame works?

So yes only that i'm considering one element either liquid of solid

25. Jul 20, 2009

### raknath

Thanks DH that helps :)