Celestia Forums

I'll probably regret asking this, but it's been bothering me most of my life, so I'll throw this in.
I sort of vaguely comprehend that an object in circular motion is under constant acceleration. Keeping Newton's 3 laws of motion in mind, what is the external force?
To take this a step further. A spinning top here on Earth will eventually stop, I assume ( deadly word) because of air and surface contact friction. If that top is spinning in ( almost) gravity free space, does it spin forever? i.e. Perpetual motion?
Thanks.... I think...
Terry Renner

"an object in circular motion is under constant acceleration. Keeping Newton's 3 laws of motion in mind, what is the external force?"

Consider one point on the rim that is accelerating. The point is accelerating in the opposite direction from a point on the opposite side of a rigid object, so the total acceleration is zero for the spinning rigid object. The external force for the first point is delivered by the rigid object that connects all sides and all points in the object. If that force is too great, the rigid object breaks apart.

"To take this a step further. A spinning top here on Earth will eventually stop, I assume ( deadly word) because of air and surface contact friction. If that top is spinning in ( almost) gravity free space, does it spin forever?"

No. There is friction in space. It is a partial vacuum, not a total vacuum. Forever is more than 999 trillion years. Maybe it would get hit by a dust particle that reverses its direction.

trenner wrote:I'll probably regret asking this...

Yes, you're about to regret it...
If you really want to brush up on your high school physics,
this is a good site: http://www.sparknotes.com/testprep/book ... er11.rhtml


BTW, not really to do with your question, but there are significant forces in space,..
Solar wind, photon pressure... (Not to mention gravity)
See for example: http://www.google.co.nz/search?as_q=sol ... =&safe=off

Aarrgh, Kzin captain wants his human slaves to do their high school exams again! Punishment for the monkey boys! The vegetarian grass eaters, they haven't even got their own gravity drives yet!

I would like to add a little to globemakers explanation, that an object in orbit also is under constant acceleration, and the external force then is gravity. I just had a thought: I suppose you could see the whole system of central and orbiting body and the curved space around them, which is causing the gravitational contraction, as a spinning top; the curved space is what is holding the structure together, too much speed and the structure flies apart. Total acceleration on the structure is now zero because the central body is not completely central, it orbits the barycenter.

With perfect circular orbits there is no energy transfer because the force is always perpendicular to the line of motion.

I think there could be one more obstacle to perpetual motion, apart from space not being completely empty: if a spinning object does not have a uniform gravitational field, -say it has a bump on its surface or a system of two or more bodies orbiting ech other-, it is supposed to send out gravity waves, and thereby it loses energy! I suppose maybe this also can be thought of as a form of friction? These gravity waves have sofar not been detected directly, only inferred by what we can see in slowing down pulsar binaries I believe.

A perfect spinning disk (or globe) would not send out such waves if it had no external couples working on it I think. Those couples would cause precession, nutation and such. Like you can see in a real spinning top. This perfect disk I think would be in perpetual motion, in a universe without external influences, no quantum fluctuations of the vacuum etc. Not sure if such a concept has any real meaning? That is a different question I suppose.

Eelco

This gives more info on gravity waves, the Wikipedia article is not yet at all clear though:
http://archive.ncsa.uiuc.edu/Cyberia/Nu ... Waves.html
http://en.wikipedia.org/wiki/Gravitational_radiation

uhhh...Ok......
I'm sort of getting it. I'm aware of the constant foot-lbs of force ( Sorry. I'm REALLY old school ) that is in centrifugal force, which I equate with what I feel when I stomp on the gas, especially going around a corner.
Where I still get a bit hung up is, me stomping on the gas is acceleration. ( and saying last rites to my gas gauge ) But if I tie a rope to my car, and tie it to a stake in the middle of the lake ( Us Canadians can do that...) and ( assuming the drive tire is the only studded tire ) go around in a circle with the accelerator held in one position, I'm not accelerating my car ! Granted, the centrifugal force sure feels like acceleration.... - but I'm not stepping on the gas !
That's a really nice web page though. I periodically wander through used book stores and buy a school physics book ( it's really interesting how the sample applications change over the years ), but only if I happen to be on my way to buy another bottle of Tylenol

trenner wrote:....Granted, the centrifugal force sure feels like acceleration....

If it looks like a duck, and it sounds like a duck, and it feels like a duck, then....

Remember that velocity has 2 components, speed and direction, so a change in either one, requires a force to be applied and hence an acceleration (in your case towards the centre of the circle) occurs.