# Please tell me why i'm wrong

1. Sep 14, 2007

### lukegregor

I've been thinking about this for a while now. I suppose I call it the Ferris Wheel. It has to do with relativity / the speed of light.

A Ferris Wheel spins...slower at it's center, and faster at it's edge. If (use your imagination) you could build a Ferris Wheel big enough, and get the central rotation up to close to the speed of light, wouldn't the outer edge of the Ferris Wheel be moving faster than the speed of light? I know that realistically it would fall apart before this ever happened...but theoretically?

Also, same concept, different question. All motion is relative, yes...so when I spin in a circle, the entire universe revolves around me. Galaxies at the edge of the universe will be stressed to complete an entire rotation around me, covering a very large distance in a very small amount of time, so they would need to be traveling faster than light.

I'm pretty sure this is flawed...can anyone tell me why?

2. Sep 14, 2007

### cesiumfrog

theoretically it would also fall apart before this ever happened.

velocity is relative, but acceleration and rotational velocity are not.

3. Sep 14, 2007

### lukegregor

why isn't rotational velocity relative? why is it excluded from relativity?

4. Sep 14, 2007

### DaveC426913

What makes you think you'd be able to get the outer edge up to the speed of light without harnessing an infinite amount of energy?

Just like you can't accelerate a spcaehip to c, you also can't accelerate a ferris-wheel rim to c. The whole axle-spokes connection is simply a red herring; it changes nothing.

5. Sep 14, 2007

### lukegregor

well yes, i'm aware of that. it's the only flaw i could think of. but, you would only need enough energy to get the central rotation up to a speed close to that of light; yes i know it would fall apart at the edge...maybe i'm just a victim of wishful thinking? overactive imagination?

6. Sep 14, 2007

### JesseM

Even theoretically, it is impossible in principle to have a perfectly rigid body in relativity. Another example of this problem is that if you had a perfectly rigid rod, so that when you pushed one end the other end moved instantaneously, this would allow you to send signals faster-than-light. Instead, relativity predicts that whenever one part of an object is accelerated, other distant parts can't react until a density wave traveling at the speed of sound from the point of the original acceleration reaches them. See here for more info.

7. Sep 15, 2007

### DaveC426913

That's not what I'm saying. I'm saying when the centre was well below the speed of light, the edge would be approaching relativistic speeds, and that would mean it would get harder and harder to accelerate the ferris wheel any faster - again, even while centre is still well below c.

8. Sep 15, 2007

### Staff: Mentor

Why do you believe that that would be true? Why doesn't the mass of the outer part matter? (hint: it does)

There is a concept called "moment of inertia" - it is the resistance of an object to rotational acceleration. It is the integrated mass distribution of an object around it's center of rotation. Ie, a mass further away from the center has a bigger impact on moment of inertia than a mass nearer to the center.

9. Sep 15, 2007

### lukegregor

cool beans...thanks everybody! forgive me, i am an (obvious) amatuer. :]

10. Sep 16, 2007

### Shooting Star

Maybe you are an amateur, but as far as the physics of a rotating disk is concerned, there is no unanimous opinion even after a century. It's called the Ehrenfest Paradox. Look it up.

11. Sep 16, 2007

### genneth

I get the impression that it's actually thought of as understood, if a little complicated. It all hinges on the fact that a rotating disc does not admit a flat metric, as should be obvious from the GR equivalence principle.

12. Sep 16, 2007

### pervect

Staff Emeritus
While there are many subtle points that are debatable about the Ehrenfest paradox (debate which is aggravated by the fact that flawed papers have slipped through the peer-review process, so that people who cite papers can find bad papers to cite), I don't think this topic is about any of them. It appears to be the usual confusion that arises from not realizing that infinitely rigid bodies are not possible in SR>

I'd suggest http://math.ucr.edu/home/baez/physics/Relativity/SR/rigid_disk.html for a popular reference on the rotating disk. Gron's paper, referenced in the above, isn't too bad, but probably too advanced, it's also not available online, you'll probably need a good (university) library.

http://scitation.aip.org/vsearch/servlet/VerityServlet?smode=strresults&query_type=search&KEY=AJPIAS&CURRENT=NO&ONLINE=YES&SMODE=strsearch&possible1zone=fpage&pjournals=AJPIAS&pyears=2001%2C2000%2C1999&page=1&origquery=&vdk_query=&chapter=0&docdisp=0&sort=rel&maxdisp=25&threshold=0&fromvolume=43&possible1=869&%5BRetrieve%5D.x=0&%5BRetrieve%5D.y=0 [Broken]

I would *not* recommend reading random references on the topic of rotating disks until one knows enough relativity to pick the good papers from the bad.

Last edited by a moderator: May 3, 2017