# Busting the myth about achieving artificial gravity by rotating a body

• B
• lolsurround
lolsurround
TL;DR Summary
TL;DR our idé of artificial gravity is flawed
Movement isn't required to achieve gravity, even the artificial kind. Standing on a rotating body requires you to be actually physically standing on it. How is this achieved? Well, by gravity of course. Unless you are tied/stuck to the rotating body, but in that case there is no gravity pushing you down on the rotating body in the first place, if you're in space.

If we assume we are stuck to the rotating body by using a pair of magnetic boots, every time we deactivate the boots we would be floating freely in space. The rotating ground below us would just fly by without us stuck to it. For centripetal for to exist there has to be gravity in the first place, thus making the whole idé of artificial gravity using a rotating a body, by nature, flawed.

davenn and PeroK
If you are on the inside of a rotating cylinder in deep space and you jump you will definitely land on the inside of the cylinder again. Because you move in a straight line (as seen in an inertial frame) and that inevitably intersects the cylinder again.

thus making the whole idé of artificial gravity using a rotating a body, by nature, flawed.
This is nonsense. The type of artificial gravity you describe is designed to work in space and there basically is no gravity, just the downward force on your body as the ring you are standing on rotates creating artificial gravity.

lolsurround said:
... the whole idé of artificial gravity using a rotating a body, by nature, flawed.
Tell that to this guy:

davenn
Ibix said:
If you are on the inside of a rotating cylinder in deep space and you jump you will definitely land on the inside of the cylinder again. Because you move in a straight line (as seen in an inertial frame) and that inevitably intersects the cylinder again.
Yes you will land on the inside again, but you're wrong about the fact that it will keep you on the floor. Even if you're moving along the cylinder in its inertial frame, you won't be pulled towards it. It will hit you again and again and you will have to hold on to it in order to keep moving along with it.

phinds said:
This is nonsense. The type of artificial gravity you describe is designed to work in space and there basically is no gravity, just the downward force on your body as the ring you are standing on rotates creating artificial gravity.
But don't you understand what is causing the downwards force that you speak of in the first place? It is centripetal force and that "force" can only exist with gravity.

lolsurround said:
you won't be pulled towards it
Depends on your frame of reference. In a rotating frame, yes you will. In an inertial frame, the floor comes up to meet you.
lolsurround said:
It will hit you again and again and you will have to hold on to it in order to keep moving along with it.
Or "friction" as we usually call it.

Dale
A.T. said:
Tell that to this guy:

lol thanks for proving my point!

lolsurround said:
But don't you understand what is causing the downwards force that you speak of in the first place? It is centripetal force and that "force" can only exist with gravity.
There is no force of gravity. That's a Newtonian myth!

Ibix
PeroK said:
There is no force of gravity. That's a Newtonian myth!
That whooshing sound you hear is that gag flying over @lolsurround's head...

davenn
Ibix said:
Depends on your frame of reference. In a rotating frame, yes you will. In an inertial frame, the floor comes up to meet you.

Or "friction" as we usually call it.
Doesn't matter what you call it. Where would your friction come from?

Ibix said:
That whooshing sound you here is that gag flying over @lolsurround's head...
Well isn't this a pleasant forum where you can have discussions with adults without making fun of each other :)

or "hear" as we usually call it...

lolsurround said:
Doesn't matter what you call it. Where would your friction come from?
The floor sliding under your feet, if you aren't travelling at the same linear velocity as the cylinder edge.

lolsurround said:
Well isn't this a pleasant forum where you can have discussions with adults without making fun of each other :)

or "hear" as we usually call it...
There's a serious point underlying it, which is that what we usually interpret as "the force of gravity" is actually the exact same kind of inertial force you're denying exists in the rotating frame case.

And you flagged this as A level (edit: I see it's now been changed to B level) meaning you claim graduate-level knowledge of this topic. You open yourself to a bit of ribbing when you make such a claim when you obviously don't have anywhere near that grasp of the topic.

russ_watters and Dale
Ibix said:
The floor sliding under your feet, if you aren't travelling at the same linear velocity as the cylinder edge.
That's where the issue is. You can't stand upright on a sliding floor under your feet unless you are pulled down on that floor. Doesn't matter how much friction you have. Let's say you have enormous friction between your shoes and the floor. Since there's no force pulling you down, how will you stay upright along that floor? Your shoes will just pull your lower body towards the direction the sliding floor is going.

lolsurround said:
Well isn't this a pleasant forum where you can have discussions with adults without making fun of each other :)
You're right that artificial gravity is, technically, a fictitious force. But, so is the real thing (the Earth's gravity is a fictitious force). One fiction replaces another.

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jbriggs444
lolsurround said:
You can't stand upright on a sliding floor under your feet unless you are pulled down on that floor.
...or if the floor somehow curves upwards and pushes up on your feet.

Is the OP actually arguing that artificial gravity by dint of rotation does not happen?

Ibix said:
There's a serious point underlying it, which is that what we usually interpret as "the force of gravity" is actually the exact same kind of inertial force you're denying exists in the rotating frame case.

And you flagged this as A level (edit: I see it's now been changed to B level) meaning you claim graduate-level knowledge of this topic. You open yourself to a bit of ribbing when you make such a claim when you obviously don't have anywhere near that grasp of the topic.
It's a stupid flag system in my view t.b.h. I don't claim to have A level knowledge, but i thought this would be interesting to discuss with someone who has graduated in physics.

OT: Isn't the force of gravity caused by mass? Not by rotating objects?

lolsurround said:
It is centripetal force and that "force" can only exist with gravity.
We have centrifuges in space. They work just fine.

Ibix
DaveC426913 said:

Is the OP actually arguing that artificial gravity by dint of rotation does not happen?
Yes and yes. That's exactly what i'm saying.

davenn
lolsurround said:
I don't claim to have A level knowledge, but i thought this would be interesting to discuss with someone who has graduated in physics.

Making unsupported claims from a lack of knowledge is not the PF way.

Ibix
lolsurround said:
Isn't the force of gravity caused by mass?
No. It's not a force. The force you feel is caused by the floor accelerating upwards into your feet, exactly the same way that the rotating cylinder floor curves upwards and pressed on your feet.

A.T. said:
We have centrifuges in space. They work just fine.

Of course we have that. I'm not arguing that centripetal force isn't a real thing. The surface tension of the blood keeps friction in the tube, causing it be pushed outside. Notice how it's tilted inside the centrifuge? It's like that for a reason.

davenn, weirdoguy and A.T.
If you examine an astronaut in a closed rotating system that has air - from an external non-rotating reference frame - you will quickly see how the astronaut is drawn toward the outer wall and contacts it after a short time.

From the astronaut's point of view, this behaves sufficiently like gravity to do its job. i.e. Keep your feet on the floor and your milk in your glass.

Where is the disconnect?

lolsurround said:
TL;DR Summary: TL;DR our idé of artificial gravity is flawed

the whole idé of artificial gravity using a rotating a body, by nature, flawed.
This is simply false. Do not repeat this assertion nor any similar false claims. The physics of centrifuges is well understood.

Start with a guy in a non- rotating centrifuge with your magnetic boots. Let the centrifuge begin rotating. The guy in the boots will feel a tug as the rotation starts, then he will start gradually feeling “weight” as the centrifuge comes to speed. Once it is at speed he can turn off the magnets and he will continue to feel weight. He can jump and he will leave the ground, feel weightless briefly, and fall back to the ground. With a sufficiently large centrifuge the Coriolis force would be small, but smaller centrifuges would have larger Coriolis forces. Those would make the fall deviate from a standard parabola, but not usually to the point of not falling back to the floor.

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lolsurround said:
I'm not arguing that centripetal force isn't a real thing.
You are. If you don't realise that's what you're doing that's the thing you need to work on understanding.

Ibix said:
No. It's not a force. The force you feel is caused by the floor accelerating upwards into your feet, exactly the same way that the rotating cylinder floor curves upwards and pressed on your feet.
https://spaceplace.nasa.gov/what-is-gravity/en/
You're wrong. The fact that gravity isn’t the same everywhere on Earth is a direct consequence of this.

weirdoguy
This is not how intelligent discussion goes on PF. Ask questions rather than make assertions.

(How long is this going to go on before the thread is locked? )

phinds
DaveC426913 said:
This is not how intelligent discussion goes on PF. Ask questions rather than make assertions.

(How long is this going to go on before the thread is locked? )
Why would it get locked for asking a legit question?

Motore, berkeman and PeroK
lolsurround said:
You're wrong.
You might want to pick a reference a little higher level than that. Masses cause spacetime curvature that lead to inertial paths that intersect the surface of the planet. The surface of the planet is what pushes you out of the inertial path - so the only force acting on you is the contact force from the planet. What we usually call the "force of gravity" is the fictitious force caused by you not being in an inertial reference frame when you're on the surface at rest.

lolsurround said:
Why would it get locked for asking a legit question?
You aren't asking questions, you're making incorrect statements.

lolsurround said:
Why would it get locked for asking a legit question?

Ibix
lolsurround said:
Why would it get locked for asking a legit question?

I see no question. I only see you claiming implicitly that physicists are wrong.

phinds
DaveC426913 said:
If you examine an astronaut in a closed rotating system that has air - from an external non-rotating reference frame - you will quickly see how the astronaut is drawn toward the outer wall and contacts it after a short time.

From the astronaut's point of view, this behaves sufficiently like gravity to do its job. i.e. Keep your feet on the floor and your milk in your glass.

Where is the disconnect?
The disconnect is that you're not getting my point. I understand what you mean but i see it as wrong. I disagree that the astronaut is drawn towards the outer wall. He starts with zero rotation together with the floor. As the floor speeds up, he holds on to it. When the floor is spinning and he let's go of it, he will not continue to be pushed towards the outer wall. He's in space and there will be nothing to draw him towards the outer wall. He will bump into the outer wall and for a while be pushed towards the floors rotational tangent, but he will not be pushed outside towards the wall.

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