# The Naked Scientists Forum

### Author Topic: Relative rotation  (Read 3101 times)

#### syhprum

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##### Relative rotation
« on: 23/10/2013 15:34:44 »
If I partially fill a bucket with water the and rotate it so that the water also rotates the water will climb up the sides of the bucket and become depressed in the centre.
why does this happen, there is no aether so what am I rotating it relative to, is it the local gravitational field of the Earth, the Solar system, the Galaxy, the whole universe or that of the sea of dark matter in which we are immersed.

#### Pmb

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##### Re: Relative rotation
« Reply #1 on: 23/10/2013 16:04:54 »
Quote from: syhprum
If I partially fill a bucket with water the and rotate it so that the water also rotates the water will climb up the sides of the bucket and become depressed in the centre.
why does this happen, there is no aether so what am I rotating it relative to, is it the local gravitational field of the Earth, the Solar system, the Galaxy, the whole universe or that of the sea of dark matter in which we are immersed.
This is explained in D’Inverno’s text on GR. The frictional effects between the bucket and the water communicate the rotation to the water inside the bucket. The centrifugal forces cause the water to pile up round the edges of the bucket and the surface becomes concave. The faster the bucket rotates, the more concave the surface becomes.

#### webplodder

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##### Re: Relative rotation
« Reply #2 on: 23/10/2013 16:09:52 »
If I partially fill a bucket with water the and rotate it so that the water also rotates the water will climb up the sides of the bucket and become depressed in the centre.
why does this happen, there is no aether so what am I rotating it relative to, is it the local gravitational field of the Earth, the Solar system, the Galaxy, the whole universe or that of the sea of dark matter in which we are immersed.

I would have thought this is due to centrifugal force. In rotating the bucket you are "throwing out", as it were, the water but the walls of the bucket prevent the water from making progress, therefore, the only path it has is to travel up the walls of the bucket and I suppose eventually, if you rotated the bucket enough would rocket up the walls of the bucket and spill out (but would it?). It is the pressure of the containment of the bucket which produces the whirlpool effect of the water which compresses it, although it's central "eye" maintains an area of calm. This is only my opinion and I stand to be corrected.
« Last Edit: 23/10/2013 16:12:13 by webplodder »

#### Pmb

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##### Re: Relative rotation
« Reply #3 on: 23/10/2013 16:26:23 »
Quote from: webplodder
I would have thought this is due to centrifugal force.
It is and you are correct. See reply #1.

#### webplodder

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##### Re: Relative rotation
« Reply #4 on: 23/10/2013 17:05:56 »
Quote from: webplodder
I would have thought this is due to centrifugal force.
It is and you are correct. See reply #1.

Thank you.

#### syhprum

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##### Re: Relative rotation
« Reply #5 on: 23/10/2013 17:17:54 »
Centrifugal force !!! what the hell is that !! rotation can only be relative are you saying the Universe is not isotropic and that there is a preferred direction against which things can be rotated I thought Einstein got rid of that idea long ago.

#### Pmb

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##### Re: Relative rotation
« Reply #6 on: 23/10/2013 17:38:52 »
Quote from: syhprum
Centrifugal force !!! what the hell is that !! rotation can only be relative are you saying the Universe is not isotropic and that there is a preferred direction against which things can be rotated I thought Einstein got rid of that idea long ago.
That’s a very confusing string of characters.

Item 1) I’m going to assume that “what the hell is that !!” is a rhetorical question and that you really know what centrifugal force is.

Item 2) There’s nothing in this thread that would suggest that the Universe is not isotropic.

Item 3) There is no preferred direction against which things can be rotated

Item 4) I thought Einstein got rid of that idea long ago. – Not at all. Einstein never got rid of centrifugal force. I don't know where you could have gotten an idea like that. In fact Einstein did the opposite. He promoted centrifugal force from a pseudo-force to a real force. He mentioned this specifically in the February 17, 1921 issue of Nature where he wrote
Quote
Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.
« Last Edit: 23/10/2013 17:44:34 by Pmb »

#### distimpson

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##### Re: Relative rotation
« Reply #7 on: 23/10/2013 18:04:13 »
the basis of the spinning liquid mirror telescopes
http://en.wikipedia.org/wiki/Liquid_mirror_telescope

#### alancalverd

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##### Re: Relative rotation
« Reply #8 on: 23/10/2013 18:15:18 »
Rotation is absolute, not relative. An object in a rotating frame must experience a force in order not to change its position within the frame, except along the singularity - the axis of rotation - where the force is zero.

#### AndroidNeox

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##### Re: Relative rotation
« Reply #9 on: 23/10/2013 22:45:27 »
If I partially fill a bucket with water the and rotate it so that the water also rotates the water will climb up the sides of the bucket and become depressed in the centre.
why does this happen, there is no aether so what am I rotating it relative to, is it the local gravitational field of the Earth, the Solar system, the Galaxy, the whole universe or that of the sea of dark matter in which we are immersed.

You might want to read about Newton's Bucket thought experiment. He makes glaring logic errors but it's interesting to think about.

Newton's Bucket:

Hang a bucket from a rope. Rotate the bucket until the rope is tightly twisted. Fill the bucket with water. Release the bucket so that the rope can untwist, spinning the bucket. Initially, the bucket will spin but the water will remain stationary; its surface flat. As the bucket continues to spin, friction between the bucket and the water will cause the water to spin. The water's surface will become concave. When the rope has completely untwisted, the angular momentum of the bucket and water will cause the bucket to continue to spin, the water to spin, and the rope to twist in the other direction. Finally, the twist of the rope will cause the bucket to stop spinning. However, for a time, the water will continue to spin; rope stationary, bucket stationary, and water spinning (with a concave surface).
The question Newton was raising was, how is rotation defined, in nature? If all motion is relative, then changes in motion can only exist as relative quantities, too. Could an object be said to spin, even if it were the only thing in the universe?

To more finely hone the experiment and eliminate unnecessary factors, Newton proposed a variation on this experiment. Suppose two rocks are bound together by a rope and set spinning in an otherwise empty universe. The question is, how can something be said to spin if it's not spinning relative to something else? The two rocks would rotate about the center of mass and the rope would be taught. But, if there's nothing else in the universe, how can the spin be defined, much less measured? What physical characteristic(s) distinguish spinning from non-spinning objects?

Newton's solution was to claim that the spin was relative to space itself. That is, empty space is the thing of reference; objects spin (or otherwise accelerate) with respect to space, regardless of whether that space contains anything else.

Other scientists have proposed other solutions. Ernst Mach proposed that, in an empty universe, 'spin' has no meaning. For spin to exist, or even be a meaningful concept, there must be matter other than the spinning object. This other matter, such as distant stars, serve as the reference for spin. The more matter present in the universe, the greater the centripetal force generated by a given mass rotating at a given speed.

So far as I know, no publications addressing this issue point out that the experiment is inherently flawed. The premises of the model are self-contradictory. Any arguments developed from Newton's Bucket Argument are invalid.

The Hole in Newton's Bucket

The Newton's Bucket Argument depends upon conservation of angular momentum. Conservation of angular momentum is the physical requirement (physical law) that the net angular momentum of any closed system (like a universe) must remain constant. By exerting force, you can change the spin of an object, but an equal and opposite spin is generated at the same time.

This is an extension of the physical law of conservation of momentum, Newton's Third Law of Motion. This is commonly phrased as, “For every action there is an equal and opposite reaction.”

It is the conservation of angular momentum that ensures that the bucket will continue to spin after the rope has unwound. It is conservation of angular momentum that causes the water to continue to spin after the bucket has stopped moving. Without conservation of angular momentum, no meaningful assertions about the experiment are possible.

However, this dependence upon the conservation of angular momentum is exactly where the entire thought experiment goes wrong.

The thought experiment depends on the assumption that you can get angular momentum from nothing.

#### Pmb

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##### Re: Relative rotation
« Reply #10 on: 23/10/2013 23:55:28 »
Quote from: AndroidNeox
You might want to read about Newton's Bucket thought experiment. He makes glaring logic errors but it's interesting to think about.
What are these glaring logic errors that you're referring to?

#### AndroidNeox

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##### Re: Relative rotation
« Reply #11 on: 24/10/2013 01:04:58 »
Quote from: AndroidNeox
You might want to read about Newton's Bucket thought experiment. He makes glaring logic errors but it's interesting to think about.
What are these glaring logic errors that you're referring to?

The experiment depends upon conservation of angular momentum. The bucket or rocks or whatever are set spinning. However, he also posits an empty universe so there's no way to get the objects spinning... there's nothing to press against.

Basically, one premise of his argument is that angular momentum is conserved however he violates that premise in the setup of the experiment. At best, the thought experiment is meaningless but I think it's an interesting problem to consider.

#### CliffordK

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##### Re: Relative rotation
« Reply #12 on: 24/10/2013 10:02:51 »
The effects of the motion of the Earth should be very obvious from looking at your bucket!!!

Consider if you're doing your bucket experiment at near the 45th parallel North.
The bucket and the water is spinning around the Earth at about 730 MPH.

In fact the gravity vector should be slightly south of the center of the earth (I think) due to the effect of centrifugal force on the bucket and the contents.  And, you'll notice the water will be slightly higher on the south wall of the bucket than on the north wall than would otherwise be expected.

Now, move our bucket to the North Pole.

You'll now discover your bucket is spinning at an extremely rapid rate of 1 Rev per day.

Now, when you look at your bucket of water (well, perhaps ethanol would be better).  The ethanol will be creeping up slightly higher on the rim of the bucket than might be expected.  Perhaps a bit higher meniscus.  If you introduced a reverse rotation of your bucket at -1 RPD, your meniscus should drop slightly.

Spinning the bucket in your hand, typically one spins it much faster, and then sees a much bigger change in levels.

Everything in the Universe is moving.
Like a moving frame, with no acceleration, the bucket, water, and everything moves at the same velocity, it is undetectable by observing your bucket of water.

You do follow curves.  The spinning of the Earth, orbit of the Earth around the sun, and the orbit of the sun around the Milky Way.  All of these curves introduce a slight acceleration.  However, all of this just slightly changes your overall gravity vector, so that it is still mostly towards the center of the Earth, but slightly greater or lower magnitude, and a slightly different direction depending on other acceleration vectors.  But, overall those changes will be hardly noticeable.

So, for all practical purposes, one should be able to consider the Earth as its own frame.

Here is a chart of the gravity due to the Earth, Sun, Milky Way, etc.

So, for example, the orbit of the Earth around the sun should change the gravity vector on your bucket by about 6mm/s2 depending on the time of day.  And the acceleration due to the orbit around the Milky Way, about 200 pm/s2.
« Last Edit: 24/10/2013 10:09:50 by CliffordK »

#### The Naked Scientists Forum

##### Re: Relative rotation
« Reply #12 on: 24/10/2013 10:02:51 »