# Rotation is absolute, linear motion is relative?

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• John Mcrain
In summary, If the box is not placed at the center of the turning table, all four walls of the box have the same acceleration vectors directions. The magnitude of the acceleration is different, but the direction is the same.
John Mcrain
Can you explain with example what mean rotation is absolute and linear motion is relative?

I don't think "absolute" is particularly well defined in this context. But usually what is meant is that you can measure acceleration in a "closed box" type experiment, but not linear velocity. If I put you in a windowless lab mounted on a turntable you can immediately detect if the turntable is rotating or not, but not whether it's in linear motion or not - in fact, there's no real meaning to "in linear motion" without reference to some state of rest.

The underlying fact is that you can always detect if you are inertial or not. If you are rotating you are not moving inertially - or at least not all of you is.

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Clair de Lune, vanhees71, russ_watters and 1 other person
Ibix said:
I don't think "absolute" is particularly well defined in this context. But usually what is meant is that you can measure acceleration in a "closed box" type experiment, but not linear velocity. If I put you in a windowless lab mounted on a turntable you can immediately detect if the turntable is rotating or not, but not whether it's in linear motion or not - in fact, there's no real meaning to "in linear motion" without reference to some state of rest.

The underlying fact is that you can always detect if you are inertial or not. If you are rotating you are not moving inertially - or at least not all of you is.
In windowless box you cant tell if box is rotating at turntable or accelerate in straight line

John Mcrain said:
In windowless box you cant tell if box is rotating at turntable or accelerate in straight line
Sure you can. The acceleration vectors point in different directions and/or have different magnitudes at different points in the rotating case but not in the linear case. With sufficiently precise measurements you could see this. If you can't do sufficiently precise measurements to spot the variation then you could get it wrong, yes.

russ_watters and Lnewqban
Ibix said:
Sure you can. The acceleration vectors point in different directions and/or have different magnitudes at different points in the rotating case but not in the linear case. With sufficiently precise measurements you could see this. If you can't do sufficiently precise measurements to spot the variation then you could get it wrong, yes.
If box is not place at the center of constant turning table, all 4 walls of box has same accelerations vectors directions like box accelerate in straight line.

I assume that men is stuck at outside wall by centrifuge, so cant move inisde box and meassure/feel that magnitude of acceleration is differnet at differnet places.

Once when G simulator reach constant RPM, men in the chair dont know if he travel in cirlce or accelerate in straight line.

Motore
John Mcrain said:
If box is not place at the center of constant turning table, all 4 walls of box has same accelerations vectors directions like box accelerate in straight line.
This is wrong. Do you know the expression for centripetal force?

russ_watters
John Mcrain said:
Once when G simulator reach constant RPM, men in the chair dont know if he travel in cirlce or accelerate in straight line.
But fluid flows in the circular canals of the ears. That detects rotation.

malawi_glenn
Ibix said:
This is wrong. Do you know the expression for centripetal force?
mxV2 / r , so at all 4 walls of box acceleration dircetions is toward center, but magnitude is different ...

Baluncore said:
But fluid flows in the circular canals of the ears. That detects rotation.
You mean ears detects synchronous rotation of men?

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John Mcrain said:
mxV2 / r , so at all 4 walls of box acceleration dircetions is toward center, but magnitude is different ...
Yes. And "towards the center" isn't the same direction at different points on a wall, unless the wall happens to be aligned radially.

In a spinning room a thrown ball will appear to take a curved path to the spinning occupants. Watch him throw the ball at 0:47 in the video below:

Ibix said:
Yes. And "towards the center" isn't the same direction at different points on a wall, unless the wall happens to be aligned radially.
How would men in chair in G simulator know if he travel in circle or accelerate in straight line?
In both case seat push his back.

John Mcrain said:
How would men in chair in G simulator know that he is travel in circle and not accelerate in straight line?
If he tosses a ball forward and it appears to curve, he would know.

malawi_glenn and berkeman
Watch at 2:58:

John Mcrain said:
How would men in chair in G simulator know if he travel in circle or accelerate in straight line?
In both case seat push his back.
Hang plumb lines and compare the angles of fall.

russ_watters
The way spaceships and many regular ships detect their orientation (and therefore rotation) is with gyroscopes, which resist changes in their rotation axis.

...but yes, accelerometers also work.

Ibix
John Mcrain said:
You mean ears detects synchronous rotation of men?
Synchronous with what?
You are assuming that a man is a solid, has no fluid content, and cannot move or turn his head.
What if he was forced against the wall by the rotation, body parallel with the axis of rotation. He then reverses the position of his head and feet. Would his ears not tell him that he was rotating in the other direction?

Copied from
https://en.wikipedia.org/wiki/Spatial_disorientation

"For example, in an aircraft that is making a coordinated (banked) turn, no matter how steep, occupants will have little or no sensation of being tilted in the air unless the horizon is visible, as the combined forces of lift and gravity are felt as pressing the occupant into the seat without a lateral force sliding them to either side. Similarly, it is possible to gradually climb or descend without a noticeable change in pressure against the seat. In some aircraft, it is possible to execute a loop without pulling negative g-forces so that, without visual reference, the pilot could be upside down without being aware of it. A gradual change in any direction of movement may not be strong enough to activate the vestibular system, so the pilot may not realize that the aircraft is accelerating, decelerating, or banking."

russ_watters
Naval gunnery officers can tell that the Earth is rotating.

See pages 449, 450, 451 and 452 in this document. This is the "Naval Ordnance and Gunnery, Volume 2, Appendix B, Part 2: Extracts from the range table for the 8"/55 gun" for the U.S. Navy. The 8"/55 was a common gun on U.S. heavy cruisers in WWII. Coriolis has an effect on both deflection (pages 451 and 452) and on range (pages 449 and 450).

This is the "toss a ball on a carousel" method, scaled up to larger and more explosive objects flung faster and farther on a more slowly rotating platform where the rotation is not always in the horizontal plane.

Then too, Foucault had an idea.

Wikipedia shows successful experiments under laboratory conditions with draining sinks. In addition, there are hurricanes, prevailing westerlies, trade winds and preferred rotation directions for high and low pressure systems. The Simpsons had a rather more humorous take on it.

Rumor [apparently false] has it that the U.K. navy suffered in the Falklands (the 1914 battle, not the 1982 conflict) from not reading the fine print in their gunnery tables. See the bottoms of pages 451 and 452 for the notes about the southern hemisphere.

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Lnewqban, russ_watters and Ibix
Baluncore said:
Synchronous with what?
You are assuming that a man is a solid, has no fluid content, and cannot move or turn his head.
What if he was forced against the wall by the rotation, body parallel with the axis of rotation. He then reverses the position of his head and feet. Would his ears not tell him that he was rotating in the other direction?
If you put bucket with water close to wall,will be water behaves identical like accelerate in straight line car?
I think will, so I said there is no difference in acceleration due to moves in circle or accelerate in straight line.

But if bucket is set at the center of G-room,, water will take U-shape.

From ground frame bucket with water set at the wall of G-room rotate around itslef because it change orientation.
If we stop rotate G-room, will be water inside bucket continued to spin for some time?

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John Mcrain said:
If you put bucket of water close to wall,will be water behaves identical like accelerate in straight line car?
It depends on how large the bucket is compared to the radius of the centrifuge. The centrifugal force is not uniform like the inertial force in linear acceleration of the car.
John Mcrain said:
But if bucket is set at the center of G-simulator,, water will take U-shape.
The water surface in the bucket placed in the cabin of the G-simulator also forms a section of that U-shape. But if the bucket is small compared to the radius of the centrifuge this might not be noticeable, and just look like a tilted plane surface.

Lnewqban
A.T. said:
The water surface in the bucket placed in the cabin of the G-simulator also forms a section of that U-shape. But if the bucket is small compared to the radius of the centrifuge it might not be noticeable, and just look like a tilted plane surface.
What casuse this U shape when bucket is at wall in Groom?From ground frame bucket with water set at the wall of G-room rotate around itslef because it change orientation.
If we stop rotate G-room, will be water inside bucket continued to spin for some time?

John Mcrain said:
What casuse this U shape when bucket is at wall in Groom?
The parabolic surface extends throughout the Groom. You can sample that curved surface with a bucket of water anywhere in the room.
https://en.wikipedia.org/wiki/Liquid-mirror_telescope

John Mcrain said:
If we stop rotate G-room, will be water inside bucket continued to spin for some time?
Yes.

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John Mcrain said:
What casuse this U shape when bucket is at wall in Groom?
The non-uniform centrifugal force field.

John Mcrain said:
What casuse this U shape when bucket is at wall in Groom?
What do you mean by "U shape"? Once the rotation axis is outside the bucket there will no longer be a dip in the middle of the bucket, sure, but the water height will still vary in a paraboloidal fashion - just a very off-center paraboloid. This is because the apparent gravity points to the center of rotation, which is at a slightly different angle to a straight wall at different points.

Ibix said:
What do you mean by "U shape"? Once the rotation axis is outside the bucket there will no longer be a dip in the middle of the bucket, sure, but the water height will still vary in a paraboloidal fashion - just a very off-center paraboloid. This is because the apparent gravity points to the center of rotation, which is at a slightly different angle to a straight wall at different points.
I assume by apparent gravity you mean the combination of Earths gravity and centrifugal force. Its direction varies because the centrifugal force is non-uniform.

A.T. said:
I assume by apparent gravity you mean the combination of Earths gravity and centrifugal force. Its direction varies because the centrifugal force is non-uniform.
Yes, agreed. I was just wondering if OP somehow thinks that we think all buckets everywhere on a rotating disk will have a minimum surface height somewhere in the middle of each bucket ("a U shape"), rather than having a small (typically off-center) part of a shared paraboloid.

Ibix said:
Yes, agreed. I was just wondering if OP somehow thinks that we think all buckets everywhere on a rotating disk will have a minimum surface height somewhere in the middle of each bucket ("a U shape"), rather than having a small (typically off-center) part of a shared paraboloid.
You mean like this?

If we stop rotate G-room when bucket is at the wall, water in bucket will still spin some time. That is proof that water also rotate about itself during G-room rotation.
Does it mean we must add together effects of water rotation(U shape) plus effects (parabolid shape ) of water circle around Groom pivot point, when G-room is rotate?

John Mcrain said:
You mean like this?
Yes.
John Mcrain said:
If we stop rotate G-room when bucket is at the wall, water in bucket will still spin some time.
It'll do a lot of sloshing around and quite possibly knock over the bucket. If it doesn't fall over the forces on the water from the decelerating bucket walls will move the rotation axis of the water to somewhere near the niddle of the bucket, yes.
John Mcrain said:
Does it mean we must add together effects of water rotation(U shape) plus effects (parabolid shape ) of water circle around Groom pivot point, when G-room is rotate?
No. Fill the room with water to a depth of a few centimetres and spin it up. The water will settle down into a paraboloid with a minimum at the rotation axis and no bulk motion of the water as seen from the room. Place a hollow cylinder, open at the ends, vertically in the water at some point. You've just made a filled bucket. Now drain all the water from everywhere except the bucket you just made. Why would the water level change? The forces on it never do.

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256bits
John Mcrain said:
You mean like this?
View attachment 320213
If we stop rotate G-room when bucket is at the wall, water in bucket will still spin some time. That is proof that water also rotate about itself during G-room rotation.
Yes if the water is at rest in the rotating frame, it has some angular momentum around the center of the bucket.

John Mcrain said:
Does it mean we must add together effects of water rotation(U shape) plus effects (parabolid shape ) of water circle around Groom pivot point, when G-room is rotate?
You can (but don't have to) decompose the inertial forces like that. But the effect of the circular translation is not a parabolid shape, because the inerial froce field from the centripetal acceleration is uniform. It's a choice of reference frame origin:

A) In the co-rotating frame with origin at the center of the room, you have the following inertial forces:
- Centrifugal force radially away from the room-center (due to rotation of the reference frame axes)

B) In the co-rotating frame with origin at the center of the bucket, you have the following inertial forces:
- Centrifugal force radially away from the bucket-center (due to rotation of the reference frame axes)
- Uniform inertial force parallel to the line connecting the room-center and the bucket-center (due to non-inertial translation of the frame origin)

The sum of the two fields in B must equal the centrifugal force in A. In both frames the water is at rest, so there are no Coriolis forces.

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Ibix said:
No.
in left case bucket is facing allways toward center (roation+circualr motion) and in right case bucket point for example allways to north(only circular motion)
How can then water in bucket behaves the same in this two cases?

A.T. said:
Yes if the water is at rest in the rotating frame, it has some angular momentum around the center of the bucket.You can (but don't have to) decompose the inertial forces like that. But the effect of the circular translation is not a parabolid shape, because the inerial froce field from the centripetal acceleration is uniform. It's a choice of reference frame origin:

A) In the co-rotating frame with origin at the center of the room, you have the following inertial forces:
- Centrifugal force radially away from the room-center (due to rotation of the reference frame axes)

B) In the co-rotating frame with origin at the center of the bucket, you have the following inertial forces:
- Centrifugal force radially away from the bucket-center (due to rotation of the reference frame axes)
- Uniform inertial force parallel to the line connecting the room-center and the bucket-center (due to non-inertial translation of the frame origin)

The sum of the two fields in B must equal the centrifugal force in A. In both frames the water is at rest, so there are no Coriolis forces.
How can centripetal acceleration be uniform, before you write centrifugal is non-uniform?
Along radius centripetal and centrifugal force must change,so they are non-uniform?

Water in bucket is not point it has lenght, so centrifugal/petal force change along this length?

John Mcrain said:
How can centripetal acceleration be uniform, before you write centrifugal is non-uniform?
Along radius centripetal and centrifugal force must change,so they are non-uniform?
- Inertial centrifugal force due to rotation of the reference frame axes is non-uniform
- Inertial force due to non-inertial translation of the reference frame origin is uniform

A.T. said:
- Inertial force due to non-inertial translation of the reference frame origin is uniform
But @Ibix write this : Fill the room with water to a depth of a few centimetres and spin it up. The water will settle down into a paraboloid with a minimum at the rotation axis and no bulk motion of the water as seen from the room.

dont understand

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