A Relativity Question

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote"><i>Originally posted by EBTX</i>
<br />Suppose there is a rapidly spinning sphere of uniform density out in intergalactic space.

Now, if it moves through space at relativistic velocities (say, relative to the backdrop of stars or, better, the microwave background radiation), the side of the sphere which is spinning in the direction of motion is travelling faster than the other side.
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Good job. You are getting along on the right track with this line of thinking. Let's take it a bit further and put it in angular momentum terms, say a rotating disk in geosyncronous orbit about the Earth with it's spin axis aligned with the Earth's rotation. Now, mathematically, we can discern that the rotational plus orbital angular momentum of the disk, measured as the sum angular momenta of each part of the disk, is different than for a non rotating disk. So in order for Kepler's Laws to be upheld (third law in particular), the orbit must change as if the mass of the rotating disk has changed. Its mass doesn't really change, but its behavior does.

An example of this is the Earth's moon which always presents the same face to the Earth. Effectively, the Moon rotates once per orbit, so that angular momentum has to be factored in when describing the Moon's behavior. Does the moon behave as if its mass were all at its center of mass? The short answer is no.

* The comments contained herein are my own and are not intended to be condescending in any way, including any presumptions about the knowledge or lack thereof of its readers.

Anyway, back to lurk mode...

James

Ok, who put "Uninflamable" signs on gasoline tanker trucks?

<blockquote id="quote"><font size="2" face="Verdana, Arial, Helvetica" id="quote">quote:<hr height="1" noshade id="quote">Hence, the side with faster velocity has "more mass"<hr height="1" noshade id="quote"></blockquote id="quote"></font id="quote">

Yeah - But does it really and truly have more mass?

Wikipedia is a good place to clear up misunderstandings about relativistic mass and angular momentum . It would be a good idea to read the " mass " article first. I suggest that you carefully read those articles and then take another look at this thread.

The most common misunderstandings ain't our fault; it's the whole d--n miseducation system. I once dated a highschool teacher who still believed, "You can't subtract a larger number from a smaller one." If you're lucky, your second grade teacher corrected that misinformation, which was drilled into your thick skull the previous year. The same method is still inflicted on us at university, where they teach outdated concepts like "relativistic mass". The few who survive the mind abuse of the intro course are disabused of it in the advanced course.

P.S.: James; gasoline trucks, cans, etc., used to have " inflamable " signs. Unfortunately, "inflamable" was its own antonym; so, in the 1950's, legislatures and Congress outlawed the word and substituted "flamable" and "nonflamable". If a truck is labeled "uninflamable", it is nonunillegal.