Angular Momentum & Artificial Gravity in a Spinning Space Station

[DARKSYDE]

im struggeling with the notion of experiencing angular momentum in a spinning space station. to me it would seem that you wouldn't "stick" to the wall/floor simply because it was spining.

for example, if you were floating in a vaccume in the center center of a tube like structure and it it began to spin around you, would you gravitate to the outter wall? from what i read its a yes. it seems to me if you were to spin with it because you were latched to a wall then you would experience the force but only until it reached its maximum velocity at which point you would return to being weightless again but now spinning with the direction of the tube.

 

[mgb_phys]

for example, if you were floating in a vaccume in the center center of a tube like structure and it it began to spin around you, would you gravitate to the outter wall?

No a spinnign object has a force but it isn;t gravity - there is no force pulling you to the wall.

you were latched to a wall then you would experience the force but only until it reached its maximum velocity at which point you would return to being weightless again but now spinning with the direction of the tube.

You would experience a force outward because you want to move in a stright line - ie off in a tangent. Because the floor is stopping you then you feel a force of the floor pushing you back, this 'feels' like gravity.

 

[DARKSYDE]

thanks for your reply. what would happen when one reached the velocity of the spinning tube? do they become weightless again? or suppose i were to jump off wall what would bring me back?

 

[mgb_phys]

You would still have some tangential velocity, but if you jumpred hard enough you would just float until you hit some other part of the structure and be spun up again.

 

[new bee]

when frends and I went to six flags. We rode a ride called spin out. This ride was a round room that spins. As it did you stuck to the wall. Then the floor lowers, with your body on the wall. Like a picture. We had stuffed animals, with us. When we through it. It would come back, so it appeared. How ever what happened, is you would get to the other side when the toy did. That was so cool until the fifth ride.

 

[DaveC426913]

thanks for your reply. what would happen when one reached the velocity of the spinning tube? do they become weightless again?

This is key:

When you spin up to the velocity of the inner wall of the space station, you do not become weightless again. To underastand why, you need to understand the source of the "force" holding you against the inner wall.

I put force in quotes because there is no force holding you against the inner wall. What is holding you there is that your inertia wants to make you go straight but the wall is constantly getting in your way.

Think about getting on a playground merry-go-round:
http://colleenanderson.files.wordpress.com/2009/08/merrygoround1.jpg

You quickly come up-to-speed but you are still being pulled hard off the merry-go-round. If you let go you'd go flying off.

You could, if you chose, lie on the merry-go-round, orienting yourself so that your feet pointed outward and your head was near the centre. You could virtually "stand" on your feet (but horizontally).

 

[qwe]

i think he was assuming that you are pulled towards the wall whether spun-up or not

 

[Ginahas]

The whole time I'm reading this I am thinking of a merry go round. So thanks for the affirmation Davecr2... But you would be pulled inward.
Thanks,
Gina

 

[DaveC426913]

The whole time I'm reading this I am thinking of a merry go round. So thanks for the affirmation Davecr2... But you would be pulled inward.
Thanks,
Gina

Inward? Why inward?

 

[ehilge]

Inward? Why inward?

To keep you going in a circle, the net force actually points towards the center of the circle. This is called a centripetal force, commonly confused with centrifugal force which is a fictitious force that you think you feel pushing you out. Without centripetal force, you would just fly off the merry-go-round. More accurately, fly in a path tangential to the merry-go-round. Hence, the centripetal force (pointing towards the center) keeping you on the merry go round.

To relate this to the the spinning space station:
When you're on the earth, gravity pulls you into the ground (weight). The ground pushes back at you with the same force as gravity (Newton's 3rd law: action and reaction) or else you would start hurtling toward the center of the earth. On the space station, I think it is analogous to think of centripetal force as the ground pushing up (towards the center) on you. And since there is something pushing you, you apply an equal and opposite force (once again, Newton's 3rd) to the rim of the station. This force is perceived as weight.

I hope that clears things up little!

 

[DaveC426913]

To keep you going in a circle, the net force actually points towards the center of the circle. This is called a centripetal force, commonly confused with centrifugal force which is a fictitious force that you think you feel pushing you out. Without centripetal force, you would just fly off the merry-go-round. More accurately, fly in a path tangential to the merry-go-round. Hence, the centripetal force (pointing towards the center) keeping you on the merry go round.

The centripetal force "pulling you inward" on a merry-go-round is simply your hand, hanging on. The moment your grip loosens you will indeed begin sliding off on a tangent.

It's kind of misleading to suggest that you would be "pulled inward".

 

[Deuterium2H]

I wanted to resurrect this thread, specifically with regards to Darksyde's initial post and question. Say you were floating weightless, inside a toroidal space station (or giant wheel like spacestation, as in 2001). Initially, nothing is rotating. Also, assume you are in a spacesuit, and that there is no atmosphere inside the torus/station.

Now, keep in mind, you are already floating motionless (with respect to the torus), and you are in the center of the torus, and are not touching or contacting any walls. And when I say the "center of the torus", I do not mean at the axis of rotation...but in the middle of the circle you would see if you took a cross section of the torus.

If the torus starts spinning, what happens to you? This was posed in Darksyde's original question, and I don't think anyone answered it correctly. My thought is that you would remain weightless, and the torus would simply spin around you. Wouldn't this be the case? I am thinking that you would only start to experience an acceleration/artificial gravity once you coupled yourself with the wall of the now spinning station...and that contact is what starts you actually rotating with the station and experiencing centripedal force and resulting "artificial gravity". Otherwise, you would just continue to be weightless and motionless with respect to the spinning station.

 

[DaveC426913]

If the torus starts spinning, what happens to you? This was posed in Darksyde's original question, and I don't think anyone answered it correctly. My thought is that you would remain weightless, and the torus would simply spin around you. Wouldn't this be the case? I am thinking that you would only start to experience an acceleration/artificial gravity once you coupled yourself with the wall of the now spinning station...and that contact is what starts you actually rotating with the station and experiencing centripedal force and resulting "artificial gravity". Otherwise, you would just continue to be weightless and motionless with respect to the spinning station.

Yup. You would remain unaffected by the rotation of the station.

Note that you specified

...that there is no atmosphere inside the torus/station.

which is why that will work. If there were any air, you would be coupled to the rotation of the station.




What's interesting to note is that you have to set up the experiment to be pretty contrived. Because you and the space station are orbiting Earth independently, it's pretty difficult to keep this setup stable. You will eventually drift into the wall.

In fact, it's almost meaningless to consider the position of the spacesuited man in any external of reference that is not the space station.

 

[Deuterium2H]

Yup. You would remain unaffected by the rotation of the station.

Note that you specified

which is why that will work. If there were any air, you would be coupled to the rotation of the station.




What's interesting to note is that you have to set up the experiment to be pretty contrived. Because you and the space station are orbiting Earth independently, it's pretty difficult to keep this setup stable. You will eventually drift into the wall.

In fact, it's almost meaningless to consider the position of the spacesuited man in any external of reference that is not the space station.

Thanks for the response, Dave.

Yes, I agree that the particular scenario I describe is "ideal", and is really meant as a thought experiment. There obviously would be subtle pertubations, differential tidal forces, that would eventually cause the astronaut to drift and contact the wall of the station. But the point is, that wouldn't be because the station began rotating. In fact, let's pretend instead of a person, the object is a small cube, say a die (as in throwing dice). And, let's also assume the walls are perfectly smooth, and there is nothing to trap the die or for the die to become "latched" to. It would take many, many impacts between the die and the rotating wall before the die eventually picked up enough angular momentum to match the angular velocity of the exterior wall...only then finally coming to rest against the wall.

 

[DaveC426913]

And, let's also assume the walls are perfectly smooth, and there is nothing to trap the die or for the die to become "latched" to. It would take many, many impacts between the die and the rotating wall before the die eventually picked up enough angular momentum to match the angular velocity of the exterior wall...only then finally coming to rest against the wall.

I'm not sure about "many, many" but a few, yes.

Note that the very first hit - even the gentlest - will destroy all stability between the die and the station. The die is now on a straight line path to another part of the wall (inner or outer), and only its imparted velocity will affect how long it takes to get there. Every subsequent impact will work to accelerate it up to station speed, which will occur more and more rapidly.

 

[Deuterium2H]

I'm not sure about "many, many" but a few, yes.

Note that the very first hit - even the gentlest - will destroy all stability between the die and the station. The die is now on a straight line path to another part of the wall (inner or outer), and only its imparted velocity will affect how long it takes to get there. Every subsequent impact will work to accelerate it up to station speed, which will occur more and more rapidly.

Yes, I concur. I probably should have typed "multiple" instead of "many, many".