# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: benm on 29/03/2019 09:28:01

Title: What would air and water do on a rotating space craft?
Post by: benm on 29/03/2019 09:28:01
Jim has an out of this world question about some Hollywood science, from the movie "Passengers":

The Avalon is an amazing space craft that uses rotation to simulate gravity. We know that air and water in a plane will move with the plane. What would air and water do on a rotating space craft? What would really to the water if somebody jumped into the Avalon's swimming pool?

Can anybody help?
Title: Re: What would air and water do on a rotating space craft?
Post by: Halc on 29/03/2019 11:53:48
Quote from: Jim
The Avalon is an amazing space craft that uses rotation to simulate gravity. We know that air and water in a plane will move with the plane. What would air and water do on a rotating space craft? What would really to the water if somebody jumped into the Avalon's swimming pool?
Water would be 'down' just like water on Earth which is attracted to Earth by gravity.  So a spinning object would have 'down' feel like the outside of the craft and 'up' be towards the center of rotation.  The closer to the edge you get, the stronger the artificial gravity.  Thus there would be no 'gravity' at the center.

The ship would have to have a huge radius for air pressure to change much from one place to another.  I climb a 4 km mountain and the air is noticeably thinner, so if the spinning craft is several km in diameter and air is not restricted by decks from flowing anywhere inside, then the pressure at the center might be somewhat lower than at the edges.
Title: Re: What would air and water do on a rotating space craft?
Post by: jimvideo on 30/03/2019 19:32:36
Water would be 'down' just like water on Earth which is attracted to Earth by gravity.  So a spinning object would have 'down' feel like the outside of the craft and 'up' be towards the center of rotation.  The closer to the edge you get, the stronger the artificial gravity.  Thus there would be no 'gravity' at the center.

Thank you for answering. This is a good description of it works in movies, but I don't believe this is correct. While gravity has an area of effect, a spinning space craft can only effect what it touches. Without gravity a thrown baseball will continue it's trajectory until it is acted on by an outside force, even if that force is air. Throw that ball in the exact opposite of the rotation and it might stay in one spot. I don't know what the answer to this question is, which is why I'm asking.
Title: Re: What would air and water do on a rotating space craft?
Post by: Bored chemist on 30/03/2019 19:55:55
If you make the ship big enough, they form oceans and an atmosphere.
Title: Re: What would air and water do on a rotating space craft?
Post by: jimvideo on 30/03/2019 20:41:43
If you make the ship big enough, they form oceans and an atmosphere.

Cool. What happens to the ocean water if I were to ride my jet ski while sending a stream of water upward?
Title: Re: What would air and water do on a rotating space craft?
Post by: Janus on 30/03/2019 22:44:12
Water would be 'down' just like water on Earth which is attracted to Earth by gravity.  So a spinning object would have 'down' feel like the outside of the craft and 'up' be towards the center of rotation.  The closer to the edge you get, the stronger the artificial gravity.  Thus there would be no 'gravity' at the center.

Thank you for answering. This is a good description of it works in movies, but I don't believe this is correct. While gravity has an area of effect, a spinning space craft can only effect what it touches. Without gravity a thrown baseball will continue it's trajectory until it is acted on by an outside force, even if that force is air. Throw that ball in the exact opposite of the rotation and it might stay in one spot. I don't know what the answer to this question is, which is why I'm asking.
If you threw the ball at exactly the right speed, then yes, you could make it standstill relative to the station axis and not "fall to the ground.  However the station would be still turning, Thus for you the person throwing the ball, it would appear that the ball was traveling in a circle at a constant distance from the floor.( ignoring any air resistance).
If you could ignore air resistance and any obstacles in the way, if you could throw a ball at ~7.9 km/sec in the horizontal while standing on the Earth, it would travel in circle around the Earth maintaining a constant distance from the ground. It would be in orbit around the Earth.   Throwing the ball at just the right velocity on the station is the equivalent putting it in orbit from the rotating station's frame of reference.*

But you can't ignore air resistance.  If add air to the station, that air will expand to fill the station, when it does so, it comes in contact with the rotating walls air molecules striking the wall will be be given some of the wall's these molecules will go on to transfer it to other molecules, etc.  Eventually, you will end up with the air in the station having the same angular velocity as the walls, and rotate with it.   If you throw your ball now, it will have to deal with the drag caused by this air which worls to drag the  ball around it with the station.  The ball will not be able to maintain is fixed position and will drift, until it hits the "floor" picking up more motion from it.

The same happens for water, it would be eventually dragged to have the same velocity as the floor.  So while there might be a bit of "settling down" to do at first, eventually you'll end up with water hugging the floor and air pressing against it.

*This only works if you throw the ball at precisely the right speed. Any slower or faster and it will drift in a straight line until is hits the curved floor of the station.
Title: Re: What would air and water do on a rotating space craft?
Post by: jimvideo on 03/04/2019 16:31:32
Turbulences in water send water in any direction. When water has upward velocity I don't see how the velocity of the space craft, the surface tension of the lake, or anything in the air can stop the water from bulging upwards or even breaking free of the lake.
Title: Re: What would air and water do on a rotating space craft?
Post by: pensador on 03/04/2019 17:27:10
Turbulences in water send water in any direction. When water has upward velocity I don't see how the velocity of the space craft, the surface tension of the lake, or anything in the air can stop the water from bulging upwards or even breaking free of the lake.

All the molecules in the space craft will be accelerated by the movement of the walls due to friction, they will then be moving in a circular path and experience a centripetal force.

The rotation of the spacecraft creates a centripetal force which acts outwards from the centre. The force will be strongest on the walls. The Heaviest object after things have settled will sink to the walls of space craft due to the centripetal force. The gradient of the force experienced due to the centripetal force will reduce towards the centre of gravity in the space craft which will be in its centre of rotation more or less.

Title: Re: What would air and water do on a rotating space craft?
Post by: jimvideo on 03/04/2019 18:47:10
I think your example is missing a step, overcoming inertia. A spinning space craft must constantly overcome inertia, which is especialy important with liquid.

When a drop of water hits a water surface another drop of water is released from the spash. On Earth gravity will bring the drop down, but in the space craft lake the drop will continue upward for several minutes. The wayword drop will continue until it hits something or dissipates.

www.maxpixel.net-Nature-Water-Drops-Of-Water-Liquid-578897.jpg (49.9 kB . 640x426 - viewed 2082 times)
Title: Re: What would air and water do on a rotating space craft?
Post by: Janus on 03/04/2019 20:43:49
I think your example is missing a step, overcoming inertia. A spinning space craft must constantly overcome inertia, which is especialy important with liquid.

When a drop of water hits a water surface another drop of water is released from the spash. On Earth gravity will bring the drop down, but in the space craft lake the drop will continue upward for several minutes. The wayword drop will continue until it hits something or dissipates.

www.maxpixel.net-Nature-Water-Drops-Of-Water-Liquid-578897.jpg (49.9 kB . 640x426 - viewed 2082 times)

But that drop of water will have the same tangential momentum it had when it it was part of the body of water.  Thus it will travel in a straight line along a vector determined by its initial inwards velocity and its tangential velocity.  Since the surface of the space ship is curved inwards as is the surface of the water, this path will intersect the water again.   For anyone moving along with the rotating craft, will also have a tangential speed that stays pretty much equal to that of the drop of water. So from their perspective, the water rises from the water and then falls back down.  There will be a small apparent drift due to Coriolis effect, the larger the "inward" velocity of the drop, the more apparent this will be.   But unless you completely remove all of the tangential motion of the drop ( effective throwing "backward" against the rotation at exactly the rotation speed), it will not fail to return to the water surface.