# 'Spinning' the truth about our own Moon?

1. Jul 2, 2007

### Techstuf

Physics Question

Applying the standard physics definition of 'spin'....If one were to take a black permanent marker and draw circles of various size in various places on the top surface of a white frisbee and throw it, would the mass inside each of those individual circles be experiencing it's own 'spin' or simply experiencing Orbital Angular Momentum?

Peace,

TS

Last edited: Jul 2, 2007
2. Jul 2, 2007

### chroot

Staff Emeritus
I'm afraid your question makes absolutely no sense. The term 'spin' has no precise scientific meaning except in quantum mechanics, and you're not discussing quantum mechanics. Futhermore, circles drawn on a spinning disc have nothing to do with orbital mechanics.

- Warren

3. Jul 2, 2007

### Techstuf

?

Only if there is no such thing as absolute truth. If all things are all relative, then I can certainly see all questions making absolutely no sense.
I'm sure that there is a modicum of sense to be found in this question. Although if I were a physics academician, I'm certain I could have phrased it less unattractively.

Given that the Physics community makes a distinction between Orbital angular momentum and angular momentum intrinsic to a spinning body, I can certainly see a relevance. Not a relative relevance. But a relevance nonethlesss.

I note that the definition, unless such has become archaic without my knowledge, defines spin pretty straightforwardly, even going so far as to distinguish between two types of Angular Momentum.

You seem to be implying that this definition is too simplisic for the purpose of analyzing heavenly bodies and their orbits?

Regards,

TS

4. Jul 2, 2007

### chroot

Staff Emeritus
What's your point, Techstuf? Drawing a circle on a rigid body like Frisbee disc doesn't make the mass inside the circle an independent body. It's just a circle on a Frisbee.

However, their motion is similar that of the Moon (or any other 1:1 tidally-locked body) in that each circle rotates once with each revolution of the disc. The relevance of this to anything, much less "truth," is unclear.

- Warren

5. Jul 2, 2007

### Techstuf

My point is a small one. I would certainly understand if you chose to acquiesce and let someone else, should one wish it, attempt to field my, how did you put it? absolutely 'no sense' question. Your earlier comment about the spinning frisbee analogy not having anything to do with celestial mechanics caught me by surprise, apparently things have evolved quite a bit since my elementary school days. I believe I was in the fifth grade when a local physics teacher used a frisbee to demonstrate a spinning body correlation to heavenly bodies. Times they are a changin.

For what it's worth, I wouldn't dream of imposing on the time of someone who might better volunteer his in a more intellectually stimulating thread. And if you chose to delete this one, I could certainly forgive your reasoning.

What I 'think' I hear Warren saying is that the mass within each circle, as per my original question, is experiencing orbital angular momentum versus angular momentum intrinsic to a spinning body.

Because, each circle certainly is not experiencing the latter, individually,

Right?

Whoever might contemplate further discussion regarding my original question, I assure you my motivation in asking it is genuine.

Regards,

TS

Last edited: Jul 2, 2007
6. Jul 2, 2007

### chroot

Staff Emeritus
The Moon has both intrinsic angular momentum (it is rotating) and orbital angular momentum (it is moving in an orbit).

- Warren

7. Jul 2, 2007

### cristo

Staff Emeritus
I don't like this definition of "classical spin"-- IMHO it is precisely this that confuses people when they come to talk about spin in quantum mechanics.

8. Jul 2, 2007

### Techstuf

Then if the Moon is experiencing both, the mass within every circle on that frisbee is as well. The frisbee as a whole is spinning according to physics. It seems paradoxical to me that circles are as well.

One can set a frisbee spinning and lightly pinch any point on it's surface, following it in it's orbit and 'feel' that point spin. But this seems to defy the very core at the definition of 'spin'. (no pun intended)

All of my constituent electrons are spinning....but I am not. Ok, maybe just my head.

My point is that as long as the Moon is part of a larger system, it's center mass is in lockstep with it's orbital angular momentum. Only upon being freed from Earth orbit, does it seem possible to begin experiencing angular momentum around it's own center of mass.

Regards,

TS

9. Jul 2, 2007

### Techstuf

Warren has stated that the mass within the circles drawn on a frisbee, like our own moon, are experiencing both intrinsic angular momentum and orbital angular momentum.

If this is so, then the tip of a mop handle swung around one's head is spinning. And all points in between.

Forgive my incredulity.

I'm having a little trouble buying it. I doubt I could afford it anyway!

Regards,

TS

10. Jul 2, 2007

### chroot

Staff Emeritus
It doesn't make any sense for mass to be 'in lockstep' with angular momentum. What you've written here is just word salad.

If you build a Foucalt pendulum on the Moon, you'll discover that it is rotating roughly once per month, and thus has angular momentum. It appears the universe has conspired to disagree with your misconceptions of physics.

- Warren

11. Jul 2, 2007

### chroot

Staff Emeritus
Misquoting me will get you nowhere. I specifically stated that the circles on your Frisbee are just that -- circles on a Frisbee.

- Warren

12. Jul 2, 2007

### Staff: Mentor

The "circles" are part of the frisbee. So how could they not be spinning? Draw an X inside each circle and watch it from above. Does not the X rotate?
Classically, the total angular momentum of any object about some point is the sum of (1) the angular momentum of its center of mass about that point, and (2) the angular momentum of the body about its center of mass.
You can consider the frisbee as a single object with just angular momentum about its center ($I_{cm}\omega$). Or you can consider the frisbee as a collection of parts (your circles) that both spin about their own centers as well as "orbit" the center of the frisbee. Add up the total angular momentum of each piece and you'll get the same answer for total angular momentum of the frisbee.

13. Jul 2, 2007

### Techstuf

Please forgive me for the misquote, Warren. You said the circles rotate, not the mass within them.

So then, you seem comfortable with my assertion that, one may be correct in determining as certain, that the tip of a mop handle being swung around one's head and all points in between it and one's hand are in fact, spinning.

It seems to me that somebody, somewhere is confusing the potential to exhibit kinetic energy via intrinsic spin with the actual act of spinning.

For if such is the case, then why distinguish between intrinsic angular momentum and orbital angular momentum at all? For if such is actually true, then they are one and the same, the distinction, moot.

TS

Last edited: Jul 2, 2007
14. Jul 2, 2007

### chroot

Staff Emeritus
Once again, "spinning" does not have any precise physical meaning. You just keep throwing words around haphazardly, indicating you have no idea what you're talking about. And yet you have the gall to claim that someone else is confused, or worse, is attempting to mislead you about high-school level physics?

- Warren

15. Jul 2, 2007

### Techstuf

Misquoting little old me, will neither earn you deferment from me as well, Warren. I never said you or anyone else was 'attempting' to mislead me, personally. Such would require malice and premeditation. I meant to infer that it seems as though there is a paradox here, and that perhaps some are being mislead. I can understand your frustration with where I'm coming from. I hope you can understand my frustration with where, it seems, you are going.

That being, in circles.

Oops, you left out that trivial little descriptor word: orbital. It seems 'word salad' is certainly a 'relative' term.

I was never good with 'virtual reality' concepts. I try, I'm just not very good with such. Given your reception to my understandably puzzling, though sincere question thus far, I shudder to think what may befall me should I visit the pinnacle of astrophysics wisdom, NASA (Never A Straight Answer).

I'd probably chicken out and stop my quest short at the Janitor's closet.

Hey Mr. Janitor, can I borrow your mop?

Warren, it's been real. In a non precise way, devoid of any physical meaning, that is.

Regards,

TS

Last edited: Jul 2, 2007
16. Jul 2, 2007

### Staff: Mentor

You seem to be trying to argue that the moon isn't rotating. You're wrong. As was pointed out, if you draw a bunch of "X"s on top of your frisbee and look at it from the top-down, you will most certainly see them rotate.

17. Jul 2, 2007

### Techstuf

Actually my intent was seeking to discover a relevant answer in keeping with the seemingly straightforward and accepted definition of the word 'spin'.

If I have an 'attitude' it is natural in response to the immaturity thus far displayed by Warren who apparently can't resist the temptation to judge another's perceived motives long enough to ascertain the fact that such a question is hardly disingenuous. And quite relevant to the layperson from whom it originated.

I am simply seeking positive discussion and call things as I see them. I was formerly not cognizant of the 'status quo' in your forum.

Now that I am more fully informed of the crystal tower hierarchy prevalent here...I'll take my leave.

I shall endeavor to keep from having your virtual door spin into my posterior as I abscond with my ignorance fully intact.

:uhh:

Regards,

TS

18. Jul 3, 2007

### Staff: Mentor

Buh bye, then....