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Author Topic: What is the Moon's velocity?  (Read 8145 times)

Offline yor_on

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What is the Moon's velocity?
« on: 27/04/2011 04:10:23 »
So the moon spins around itself and around Earth, right?

Okay, what is its velocity?
« Last Edit: 27/04/2011 08:05:28 by chris »


 

Offline yor_on

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Re: What is the Moon's velocity?
« Reply #1 on: 27/04/2011 04:11:02 »
Be suspicious...

Be very suspicious :)
 

Offline burning

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Re: What is the Moon's velocity?
« Reply #2 on: 27/04/2011 04:54:09 »
OK, I'll bite.

Before anyone can answer the question, you need to define the frame of reference you're interested in and possibly specify the time as well.

In the absence of any further information, I'll choose a frame of reference fixed relative to the moon and say that the moon's velocity is zero. ;)
 

Offline Geezer

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Re: What is the Moon's velocity?
« Reply #3 on: 27/04/2011 05:38:39 »
OK, I'll bite.

Before anyone can answer the question, you need to define the frame of reference you're interested in and possibly specify the time as well.

In the absence of any further information, I'll choose a frame of reference fixed relative to the moon and say that the moon's velocity is zero. ;)

It's zero relative to the Earth too (on average).

 

Offline yor_on

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What is the Moon's velocity?
« Reply #4 on: 27/04/2011 15:07:32 »
I know I said suspicious , but  t h i s suspicious :)
Darn, you're both right, but why?

Why doesn't the moon have a velocity?
 

Offline burning

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What is the Moon's velocity?
« Reply #5 on: 27/04/2011 18:01:26 »
I know I said suspicious , but  t h i s suspicious :)
Darn, you're both right, but why?

Why doesn't the moon have a velocity?

Well, I wouldn't say it doesn't have a velocity.  We can pick a frame of reference in which its velocity is always zero (my "solution").  We can pick a frame of reference in which its velocity averages to zero but in which the instantaneous velocity is never zero. (Geezer's "solution").  We can pick a frame of reference in which at a specific instant it has a zero velocity but in which it will not in general have a zero velocity.  And we can pick a frame of reference in which the instantaneous velocity is never zero and does not average to zero.

So what is it you have in mind that leads you to say that the moon doesn't have a velocity, period?
 

Offline yor_on

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What is the Moon's velocity?
« Reply #6 on: 27/04/2011 19:27:03 »
Well, velocity is a vector right? and a vector can't really be seen to be spinning around?
As I said, I was trying to be tricky there but it backfired :)

Then again, does 'circular motion' exist in our SpaceTime? Newton didn't believe so as I understands it and Einstein seem to have defined SpaceTime as 'warped', all bends and curves being a direct effect of gravity?

And then we have the definition you introduced Burning.
Clever clever :)

But it seems also a question of absolutes?
What defines a circle?

Is it a infinite amount of minuscule straight lines, is it 'pointlike' indentions, interpreted by times arrow? Or does both shapes exist?

And can you turn it around? Could I say that a straight line is conglomerate of bent lines, too small to be observed by our measurements?

I know :)
Pure speculation.
 

Offline Geezer

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What is the Moon's velocity?
« Reply #7 on: 27/04/2011 20:13:48 »
Right. Velocity is a vector, so it has magnitude and direction. But the Moon's direction is continuously changing relative to the Earth, so you can only talk in terms of its velocity at a particular instant in time, and if you keep summing up all the instantaneous velocities, the result always tends towards zero.

Don't ask me to define what an "instant" is, but I think it's fairly small.
 

Offline granpa

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Offline Geezer

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What is the Moon's velocity?
« Reply #9 on: 27/04/2011 20:54:52 »
 

Offline Democritus

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What is the Moon's velocity?
« Reply #10 on: 30/04/2011 16:14:05 »
yor_on asks "What defines a circle? Is it an infinite amount of minuscule straight lines, is it 'pointlike' indentions, interpreted by times arrow?"

This is a good question, well better, it's a great question. I recall that decades ago as a high school student in maths class being asked to define the circle. Waking from my daydream I said that a circle is a regular polygon with an infinite number of sides. This response was not well received by any present...wasn't in the book. You get that.
 

Offline Bored chemist

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What is the Moon's velocity?
« Reply #11 on: 30/04/2011 23:53:33 »
"yor_on asks "What defines a circle?!
The locus of points in a plane equidistant from a specified point.
 

Offline thebrain13

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What is the Moon's velocity?
« Reply #12 on: 02/05/2011 08:11:40 »
You could set its' velocity relative to the cosmic microwave background radiation. If one were looking for a possible "preferred frame of reference" to establish velocity, I'd start there.
 

Offline yor_on

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What is the Moon's velocity?
« Reply #13 on: 03/05/2011 04:12:27 »
Yes, then you have the 'fact' as i understands it, that in nature there are no straight lines to be found. I can't swear to it though, read it somewhere a long time ago and it stuck in my mind. BC says points as defined in a plane equidistant from a specified point. And Democritus suggest polygons, having a infinite number of sides.

So how would those ideas define a sphere?
 

Offline yor_on

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What is the Moon's velocity?
« Reply #14 on: 03/05/2011 04:24:37 »
I would expect the 'preferred velocity' of a circular motion to become a infinite amount of straight lines myself Brain :) Ending as they start. Then again, there is always a possibility to see all circular motions in SpaceTime as being 'straight lines', when excluding gravity, if we accept that interpretation. But I can't see it myself, thinking about it, though.

Maybe there is some mathematical translation somewhere on the net? Preferably with a visual representation too :)It would be interesting to see what the moons orbit would translate into without gravity and 3-D, if you see how I mean here?

==

On the third tentacle, isn't all orbits accelerations?
I'm not sure how Einstein defined it suddenly?

The Newtonian definition is a constant 'acceleration', right. But isn't it also a geodesic, and therefore according to the theory of relativity, the 'easiest', least energy consuming path, gravitationally speaking?

I need too look at that. Both ideas make sense to me, but they clash.
=

No they don't clash. They are mirrors to each other. Newton thinks of it as a 'flat' space, and Einstein of a 'curved/distorted'. In a flat space a orbit must be represented of a willingness to move in a straight line, like rotating with a ball on a string, letting it go. But then we have 'gravity' that forms 'space' and if we could color its gradients depending on 'gravity strength' we would have a lot of shades representing the way those gradients bend/distort. But I still find it really hard to translate a orbit into a straight line mentally.

How the he* would it look? Ignoring relative motion, unfolding gravity.
« Last Edit: 03/05/2011 07:11:57 by yor_on »
 

Offline yor_on

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What is the Moon's velocity?
« Reply #15 on: 03/05/2011 07:23:19 »
Maybe you can't? In Einsteins universe time is a intrinsic component. In a universe where time is a free component that you can add or take away as needed the orbit representing a straight line becomes a impossibility as you have a closed string. Introducing time there is no such thing though, in such a universe it becomes spirals instead. Think of a flat plane where you have one big circle in the middle representing Earth and a little circle that orbits it (the moon). Now you will be 'time' and take that flat plane and move it, forward towards you or away from you wont matter. Imagine now this moons motion through space as you move the whole 'flat plane', all included. It becomes a spiraling motion. So, in Einsteins universe there can't be any closed strings? Not as long as we expect a arrow of time.

O0o0uch :)

==

And from that spiraling motion to a straight line I expect it to become simpler. But that also state, if I'm right here, that 'SpaceTime' can't be described without a time component. So?
==
I better get some sleep huh :)

==

I can't help it, I'm fascinated by the relationship between geometry and SpaceTime. And some of you might feel the same? Democritus, perhaps? Anyway ,I found this very nice book, it's somewhat old so you will find it in any well stocked library I hope. Geometry And The Imagination By Hilbert Cohn-Vossen. Lots of nice visulals helping one see the concepts. A must read for anyone wanting to understand all those twisting your SpaceTime :)  And yes, It's weird in its own way, but so far it seems logical to me, and you can actually understand the words :) No small feat I think. Even Einstein should have been pleased reading it. Geometry and the imagination. Excerpts from Google Books.
« Last Edit: 03/05/2011 20:11:38 by yor_on »
 

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What is the Moon's velocity?
« Reply #15 on: 03/05/2011 07:23:19 »

 

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