# Gravity On a Ring Around the World

1. Dec 2, 2008

### pirtle

Alright, suppose you managed to build some type of massive ring around the Earth; concentric to the Earth, that is. I'm picturing a continuous metal pole-type object, bigger than Earth and its atmosphere, suspended around the earth like a ring of a gyroscope.

What would the effect of Earth's gravity be on this ring? Would it simply sit there? Or would the ring spin or orbit around the Earth?

2. Dec 2, 2008

### Math Jeans

Unless the ring is made up of something less dense than space, then I believe it would have to orbit.

3. Dec 2, 2008

### mgb_phys

If it was uniform and centred on the Earth's centre of mass would it have to orbit ?
It would be in a (admittedly very unstable) equilibrium.

4. Dec 2, 2008

### Math Jeans

Assuming that the ring is solid. I was thinking more along the line of gasses...but that works too.

5. Dec 2, 2008

### Hurkyl

Staff Emeritus
Assuming the material was strong enough to avoid breaking and falling into the Earth... surely it would be forced to begin rotating via tidal forces (both from irregularities of the Earth's surface, and from the moon) and friction from the Earth's atmosphere.

6. Dec 2, 2008

### Math Jeans

Well, there is always some place that a solid ring capable of not falling in can sit with respect to the earth in which the forces from gravity would cancel out all other forces (tidal and friction), however, that, once again, requires a very very stable equilibrium, and it would be much easier to assume that it just spins.

7. Dec 2, 2008

### Hurkyl

Staff Emeritus
I don't believe that. At the very least, the ring should begin rotating fast enough to be geostationary.

8. Dec 2, 2008

9. Dec 2, 2008

### Math Jeans

I believe the shape in question was a ring. Not a sphere. Also, the concept of a Dyson sphere is focused on a completely different problem.

10. Dec 2, 2008

### mgb_phys

I was assuming a http://en.wikipedia.org/wiki/Ringworld type ring - You would have to get rid of the moon of course.

I think the theory says that saturn type rings are only stable if you have shepard moons.

Last edited: Dec 2, 2008
11. Dec 2, 2008

### pirtle

Hah. Yes much more like Ringworld than a Dyson Sphere.
How would a ring that encompasses the Earth orbit around the Earth?
Would it simply start spinning? Or would it rotate such as the outer ring of a gyroscope does when its standing on its pin?

12. Dec 2, 2008

### mgb_phys

It wouldn't notice that we are going around the sun - it wold go along with the earth jst like the moon does.
A uniform ring wouldn't start spinning because there is nothing for tidal forces to act on.
If you didn't do anything it would always face in the sam edirection compared to distant stars and so would rise and set over each point on earth in the same way the sun does(although slightly slower by 4mins/day)

13. Dec 2, 2008

### NoTime

The article covers a wide variation of the theme, not limited to the initial Dyson concept.
A ring is just a slice out of a sphere and the ring subclass is referred to as a niven ring.
Other than that anything else is a matter of scale.

14. Dec 2, 2008

### Janus

Staff Emeritus
A solid ring could not maintain a stable orbit, spinning or not. The slightest disturbance would cause it to eventually drift "off-center", and from there it will continue to drift until it brushed against the Earth. James Maxwell proved this to be true for any solid ring around a planet, thereby determining that Saturn's rings could not be solid.

15. Dec 3, 2008

Staff Emeritus
Thank you, Janus. I was reading this and was hoping that someone would chime in.

There is no gravitational force from a ring on an object inside it (in the plane of the ring). Therefore the ring exerts no net gravitational force on the earth, so the earth can exist no net gravitational force on the ring.

16. Dec 3, 2008

### pirtle

Awesome! Thanks for taking the time to answer my question.

I didn't think of looking up informations about Saturn and it's rings. That's very interesesting.