# Centrifugal Force as Artificial Gravity

#### Electric Skeptic

##### Well-known member
We're probably all familiar with the idea of a spaceship (with at least some parts) rotating, to produce artificial gravity. There's that memorable sequence in 2001 of Poole running around the loop, for example.

In Rendezvous With Rama, Arthur C. Clarke has a miles long spaceship that is a huge hollow cylinder (imagine a big oil drum, but miles long). It rotates to produce artificial gravity on the inside of its hull.

Part of the consequent physics he describes is that at the axis of rotation, there is zero gravity (I'll stop using 'artificial' all the time; take it as read) and as one 'descends' toward the hull of the ship, the gravity increases. At one stage he has a character pedalling a user-propelled aircraft (that is designed, if memory serves, to function in Martian gravity, which is about 1/3 of earths). The character can't fly right at the axis, as there is no gravity at all - his craft won't work, it can't even work out which way is 'up'. So he has to 'descend' a bit until he encounters gravity approximately the same as that for which his craft is intended.

Am I wrong, or is this entire premise flawed? If you imagine a spinning cylinder in a no-gravity field, there would, just like in Clarke's construct, be gravity if you were on the inside of the hull. But would there be a force pulling you down if you were not in contact with the hull? The centrifugal force would only affect that which is in contact with the hull...wouldn't it?

Would it make a difference if the cylinder were filled with air (or some gas) as opposed to being a vacuum?

#### puddleglum

##### Well-known member
When an object is in motion inertia will make it move in a straight line unless some other force intervenes. Inside a rotating spaceship would prevent you form continuing in a straight line but would cause you to move in the direction of rotation. The effect would be the same as if gravity were attracting you toward the outside of the ship.

If the spaceship were filled with air I assume the molecules would be affected in the same way you are.

#### LifeIn

##### Well-known member
In Rendezvous With Rama, Arthur C. Clarke has a miles long spaceship that is a huge hollow cylinder (imagine a big oil drum, but miles long). It rotates to produce artificial gravity on the inside of its hull.
I like how they figured out the likely maximum deceleration they might experience based on the difference between the liquid level in the cylindrical sea and the level of the "shore". That whole story was a nice exercise is extrapolating simple laws of physics onto a fantastic scenario. (They always do everything in threes.)

Part of the consequent physics he describes is that at the axis of rotation, there is zero gravity (I'll stop using 'artificial' all the time; take it as read) and as one 'descends' toward the hull of the ship, the gravity increases. At one stage he has a character pedalling a user-propelled aircraft (that is designed, if memory serves, to function in Martian gravity, which is about 1/3 of earths). The character can't fly right at the axis, as there is no gravity at all - his craft won't work, it can't even work out which way is 'up'. So he has to 'descend' a bit until he encounters gravity approximately the same as that for which his craft is intended.

Am I wrong, or is this entire premise flawed? If you imagine a spinning cylinder in a no-gravity field, there would, just like in Clarke's construct, be gravity if you were on the inside of the hull. But would there be a force pulling you down if you were not in contact with the hull? The centrifugal force would only affect that which is in contact with the hull...wouldn't it?

Would it make a difference if the cylinder were filled with air (or some gas) as opposed to being a vacuum?
That's a good question. Let's see. The pedal craft is generating a small about of lift, which acts perpendicular to the wings and path through air. And it is just enough to maintain a fixed distance from the axis. The guy steering the craft is presumably following landmarks on the hull that he can see. So he is constantly turning the craft to exert lift in the direction of the axis of rotation. It is not automatic. It requires a conscious effort on the part of the pilot to fly what will turn out to be a spiral path. But that is exactly what he would do if he was in visual contact with the hull, especially since the ship is rotating so slowly.

But as I think of it more, there is another way the pedal craft pilot (I forget his name now too) could fly parallel to the axis. Instead of following a line of longitude on the hull, he could fly in a straight line parallel to the axis of rotation and just allow the hull to rotate under him. This would mean he would experience zero gravity (since he is no longer rotating) and would have to trim the craft for zero lift, the same as if he were flying exactly on the axis of rotation. (As a private pilot myself, I don't see that as a problem.) The only kink in that approach is that due to friction, the air in the vessel as by now stabilized into a rotation with hull. So the closer he flew to the hull, the greater crosswind he would experience. But if the crosswind is to too bad, a little bit of crab angle would correct for it.

#### Temujin

##### Well-known member
I like how they figured out the likely maximum deceleration they might experience based on the difference between the liquid level in the cylindrical sea and the level of the "shore". That whole story was a nice exercise is extrapolating simple laws of physics onto a fantastic scenario. (They always do everything in threes.)

That's a good question. Let's see. The pedal craft is generating a small about of lift, which acts perpendicular to the wings and path through air. And it is just enough to maintain a fixed distance from the axis. The guy steering the craft is presumably following landmarks on the hull that he can see. So he is constantly turning the craft to exert lift in the direction of the axis of rotation. It is not automatic. It requires a conscious effort on the part of the pilot to fly what will turn out to be a spiral path. But that is exactly what he would do if he was in visual contact with the hull, especially since the ship is rotating so slowly.

But as I think of it more, there is another way the pedal craft pilot (I forget his name now too) could fly parallel to the axis. Instead of following a line of longitude on the hull, he could fly in a straight line parallel to the axis of rotation and just allow the hull to rotate under him. This would mean he would experience zero gravity (since he is no longer rotating) and would have to trim the craft for zero lift, the same as if he were flying exactly on the axis of rotation. (As a private pilot myself, I don't see that as a problem.) The only kink in that approach is that due to friction, the air in the vessel as by now stabilized into a rotation with hull. So the closer he flew to the hull, the greater crosswind he would experience. But if the crosswind is to too bad, a little bit of crab angle would correct for it.
All we need now is for Elon Musk to fund it.

#### inertia

##### Super Member
We're probably all familiar with the idea of a spaceship (with at least some parts) rotating, to produce artificial gravity. There's that memorable sequence in 2001 of Poole running around the loop, for example.

In Rendezvous With Rama, Arthur C. Clarke has a miles long spaceship that is a huge hollow cylinder (imagine a big oil drum, but miles long). It rotates to produce artificial gravity on the inside of its hull.

Part of the consequent physics he describes is that at the axis of rotation, there is zero gravity (I'll stop using 'artificial' all the time; take it as read) and as one 'descends' toward the hull of the ship, the gravity increases. At one stage he has a character pedalling a user-propelled aircraft (that is designed, if memory serves, to function in Martian gravity, which is about 1/3 of earths). The character can't fly right at the axis, as there is no gravity at all - his craft won't work, it can't even work out which way is 'up'. So he has to 'descend' a bit until he encounters gravity approximately the same as that for which his craft is intended.

Am I wrong, or is this entire premise flawed? If you imagine a spinning cylinder in a no-gravity field, there would, just like in Clarke's construct, be gravity if you were on the inside of the hull. But would there be a force pulling you down if you were not in contact with the hull? The centrifugal force would only affect that which is in contact with the hull...wouldn't it?

Would it make a difference if the cylinder were filled with air (or some gas) as opposed to being a vacuum?

I'm thinking that a representative model would be a hoop with spokes where the moment of inertia through the center of mass is: Icm = Ma^2.

So, at a = 0 there is no moment of inertia, but along one of the spokes, the centrifugal force due to the rotational motion of the coordinate system will be perpendicular to the axis and directed outward. Indeed it would act as an "effective gravity". A fluid such as air would form a parabolic shape along the inner surface of the hoop's outer hull and the air density would make a difference.

So, a series of hoops within the outer hoop with one designed to produce an effective gravity similar to Mars should work. The propellers of a self-propelled aircraft would have to be stiff and rotate a lot faster than in higher-density environments as we learned from recent flight tests on Mars.

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inertia

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#### 4tune8chance

##### Active member
The atmosphere makes all the difference, down on the 'ground' the air is moving with the floor of the cylinder, while at the axis it is not. When things change, i.e. the rotation is changed or in the book the temperature inside changes then the air needs to re-stabilise, in the book as the cylinder warmed and the air heated a cylindrical storm formed. It settled down when a new equilibrium was achieved.

#### Electric Skeptic

##### Well-known member
The atmosphere makes all the difference, down on the 'ground' the air is moving with the floor of the cylinder, while at the axis it is not. When things change, i.e. the rotation is changed or in the book the temperature inside changes then the air needs to re-stabilise, in the book as the cylinder warmed and the air heated a cylindrical storm formed. It settled down when a new equilibrium was achieved.
So how would it work in a vacuum?

#### 4tune8chance

##### Active member
So how would it work in a vacuum?
In a vacuum. lets start with everything at rest, in space, Zero G, and the glider along the axis. If the Cylinder starts moving or rotating the glider will not be affected (almost). Worst case scenario the glider is impacted by the inside of the cylinder wall. There is a small gravitational attraction between glider and cylinder but this can be ignored for the sake of simplicity.

Note, that the same thing will happen even with an atmosphere if the cylinder accelerates extremely fast. This is a craft that requires slow acceleration and slow attitude changes. If you remember the novel, this effect is seen when an attitude change happens, all the robots return to the sea, the ship goes into lockdown, and during the change, huge waves, and a strange waterfall is created as all the fluids reach the new equilibrium.

#### inertia

##### Super Member
In a vacuum. lets start with everything at rest, in space, Zero G, and the glider along the axis. If the Cylinder starts moving or rotating the glider will not be affected (almost). Worst case scenario the glider is impacted by the inside of the cylinder wall. There is a small gravitational attraction between glider and cylinder but this can be ignored for the sake of simplicity.

Note, that the same thing will happen even with an atmosphere if the cylinder accelerates extremely fast. This is a craft that requires slow acceleration and slow attitude changes. If you remember the novel, this effect is seen when an attitude change happens, all the robots return to the sea, the ship goes into lockdown, and during the change, huge waves, and a strange waterfall is created as all the fluids reach the new equilibrium.

Thinking...

If the craft is in a vacuum within a spinning cylinder and completely unattached to the cylinder's structure where:

- both the cylinder and the craft do not have any other velocity components
and
- gravity from the cylinder's mass is negligible
and
- the Earth is too far away for its gravity to count
and
- an observer on the craft considers the craft to be at rest ( he tests this using a pendulum on the craft )

a "pseudo force" such as the centrifugal force will appear in the accelerated frame only.

___

#### 4tune8chance

##### Active member
Thinking...

If the craft is in a vacuum within a spinning cylinder and completely unattached to the cylinder's structure where:

- both the cylinder and the craft do not have any other velocity components
and
- gravity from the cylinder's mass is negligible
and
- the Earth is too far away for its gravity to count
and
- an observer on the craft considers the craft to be at rest ( he tests this using a pendulum on the craft )

a "pseudo force" such as the centrifugal force will appear in the accelerated frame only.

___
I don't see how a pendulum would even work (on the craft), as the Cylinder and Craft (assume it is not resting on the Cylinder) are not attached. The craft would behave on the inside the same way it would as if it were on the outside. How does a force that is applied to the cylinder (attitude or position) transfer itself to the craft. It would need an atmosphere or be resting on the wall. Imagine a marble in a tube then shake the tube, same sort of dynamics.
Consider a car before seatbelts, car stops suddenly, person is launched through the windscreen. Put on seatbelt, or fill the car with a thick fluid like water, and the passenger will be spared (somewhat).

#### inertia

##### Super Member
I don't see how a pendulum would even work (on the craft), as the Cylinder and Craft (assume it is not resting on the Cylinder) are not attached. The craft would behave on the inside the same way it would as if it were on the outside. How does a force that is applied to the cylinder (attitude or position) transfer itself to the craft. It would need an atmosphere or be resting on the wall. Imagine a marble in a tube then shake the tube, same sort of dynamics.
Consider a car before seatbelts, car stops suddenly, person is launched through the windscreen. Put on seatbelt, or fill the car with a thick fluid like water, and the passenger will be spared (somewhat).

Yep - a pendulum attached to the craft wouldn't demonstrate any acceleration. It would act as a null-result test.

This is fun ---> springs and pendulums on the ISS

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#### 4tune8chance

##### Active member
Yep - a pendulum attached to the craft wouldn't demonstrate any acceleration. It would act as a null-result test.

This is fun ---> springs and pendulums on the ISS

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Indeed. Arthur C. Clarke is my favourite author. Andy Weir, IMO, coming hot on his heels putting some serious science in the science fiction.

#### Electric Skeptic

##### Well-known member
Thinking...

If the craft is in a vacuum within a spinning cylinder and completely unattached to the cylinder's structure where:

- both the cylinder and the craft do not have any other velocity components
and
- gravity from the cylinder's mass is negligible
and
- the Earth is too far away for its gravity to count
and
- an observer on the craft considers the craft to be at rest ( he tests this using a pendulum on the craft )

a "pseudo force" such as the centrifugal force will appear in the accelerated frame only.

___
That was my thinking originally; I neglected to account for the moving atmosphere in the novel.

#### Electric Skeptic

##### Well-known member
Indeed. Arthur C. Clarke is my favourite author. Andy Weir, IMO, coming hot on his heels putting some serious science in the science fiction.
I have long loved Arthur C. Clarke's work. Although the Rama series got decidedly stranger the further it went.

I have always considered Childhood's End the finest science fiction novel I've ever read. I never understood why it's not more widely recognised/praised.

#### 4tune8chance

##### Active member
I have long loved Arthur C. Clarke's work. Although the Rama series got decidedly stranger the further it went.

I have always considered Childhood's End the finest science fiction novel I've ever read. I never understood why it's not more widely recognised/praised.
Yes the Rama series from 2 and on was co-authored. I recently watched the TV version of Childhoods End, quite well done. I don't think Arthur C Clarke has a single bad book, i'v enjoyed every one.

#### Electric Skeptic

##### Well-known member
Yes the Rama series from 2 and on was co-authored. I recently watched the TV version of Childhoods End, quite well done. I don't think Arthur C Clarke has a single bad book, i'v enjoyed every one.
I didn't know there was a TV version! I will look for it.

Found it - so it's three episodes, about 90 minutes each? Wonderful. And with Colm Meaney - one of my favourite actors.

#### Whateverman

##### Well-known member
Am I wrong, or is this entire premise flawed?
It's not flawed.

You can prove it by going to your neighborhood playground, jumping on the great merry-go-round that spins horizontally to the ground. The kind kids speed up by kicking their legs against the ground as it spins:

Once it's spinning pretty fast, walk to the center and see what it feels like. Then walk to to the edge and do the same.

"Gravity" increases the further away from the hub you are.

#### Whateverman

##### Well-known member
Incidentally, isn't this concept also central to Larry Niven's Ringworld? It's been a long time since I read it...

#### Whateverman

##### Well-known member
I have long loved Arthur C. Clarke's work. Although the Rama series got decidedly stranger the further it went.

I have always considered Childhood's End the finest science fiction novel I've ever read. I never understood why it's not more widely recognised/praised.
It's pretty good.

I wont turn this thread into a "favorite sci-fi book" fest, but if we're listing good books that should have been more influential, I'd point to Asimov's Foundation series, as well as Julian May's Saga of Pleistocene Exile. The latter is possibly my all-time fav.

#### 4tune8chance

##### Active member
Incidentally, isn't this concept also central to Larry Niven's Ringworld? It's been a long time since I read it...
Indeed, but on a massive scale. Whole topic about the stability of such a structure.