# Artificial gravity? what's up with that?

(First of all you should know that my grammar and spelling is not the best. Try to understand that it’s difficult to me to expose this questions).

As I watched the two spaceships docking in rhythm with the Blue Danube Waltz in “2001: A Space Oddessey” something occurred to me:

In theory, you can simulate gravity in space by spinning a ship around it’s axis and taking advantage of the centrifugal force created by this motion. But in the movie, there comes a point when two ships are spinning in a syncronized way; in a way that to a passenger on either ship, none seem to be spinning at all. If you remove all other reference points in the universe, how do you know that they are in fact spinning? The passengers still feel the gravety effect (or do they?); does this mean that there is an absolute angular velocity for a body no matter what the reference system is? How do you know which is the correct reference system to take into account to measure this velocity?

Another tricky question about artificial gravity: if I am in a cylindrical spaceship, spinning so that there is “gravity”, what would happen to a baseball that I tried to toss to a person on the ther side of the ship?

One last doubt: If you put one of those airport moving walkways on one of these ships, that just happens to go at the same speed as the spin of the ship, but in the opposite direction, would a space traveler, getting on this walkway trying to get to his gate on time, all of a sudden fly of the walkway? Does the centrifugal effect should be cancelled by the speed of the walkway?

In conclusion: What’s the deal with artificial gravity?

We don’t have it yet. Everything else is just mucking around with centrifugal force.

You know they are spinning because the movement is still holding you to the deck. Beyond that, I dunno. I’m sure they have ways of figuring it out. Of course, reference points are everything.

No idea on the baseball, but walking against the spin wouldn’t throw you off, it would actually kind of serve to make you heavier. however, it would be much faster than going the other direction, because not only are you moving, your destination is moving towards you.

This is just practical thinking, someone with knowledge please save us!

The baseball would travel in a straight line from the moment it left your hand until it hit the wall. To you, spinning around on the inside surface, it would look like a somewhat strange arc, hitting the wall in a place you probably wouldn’t expect. To an outside observer, the ball travelled in a straight line. This neglects the effects of air inside the cylinder. You can demonstrate this on a Roundup ride at a carnival. Just don’t let the attendant catch you getting on the ride with a tennis ball.

You can tell when you’re spinning because you are constantly accelerating. This is different than sitting at the bottom of a gravity well, in a non-accelerating field. Things like throwing a ball would let you know that you are moving. There are many other ways but I’ll let a physicist do a better job of explaining.

Whether or not you have a VISIBLE reference is unimportant. Covering your eyes doesnt change the fact that light surrounds you. A person in such a position may have a hard time figuring out why there is “gravity” in his system if he can not show that he is spinning but it would not effect the fact that he is. Really simply throwing a ball in the air will reveal the spin anyway.

if you throw the ball at a right angle to the direction of the spin you will notice a difference in its flight as the ball will lose ground in relation to the ships rotation and will be to one side of the person you are throwing to depending on the direction of the spin. If you throw the ball in the same direction or opposite direction of the spin and cross the spin axis you will definitly see a deflection in the path. at lower heights you would still observe the effect of “precession”. the ball will move in a curve

One last doubt: If you put one of those airport moving walkways on one of these ships, that just happens to go at the same speed as the spin of the ship, but in the opposite direction, would a space traveler, getting on this walkway trying to get to his gate on time, all of a sudden fly of the walkway?

In conclusion: What’s the deal with artificial gravity?

The deal is that it doesn’t exist yet. we can simulate it but we can’t induce it.

Regarding the walkway question, if you were on such a walkway, you would experience no force pushing you downward. In effect, the walkway is a ring that remains stationary (nonrotating), so, if you were standing on it, it would be as though the station weren’t rotating at all. If you were to push off of it over the rotating portion, you’d still feel no gravitation effects, and would move in a straight line. If you adjusted your direction (with little jets or something) so that you collided with the rotating portion of the “ground”, you’d tumble as you hit, like someone being pushed out of a car (unless you and the ground are somehow totally friction-free, in which case I think you’d just bounce back). On hitting the ground, it will shove you in the direction tangent to the spot where you hit the ground. But since the ground curves around the center, this just pushes you back into the ground to be shoved again, and again, etc., until your rotational velocity matches that of the ground. At this point, you’ll experience full “gravity”.

Hope I got that right. Hopefully someone will be along with some equations to settle the matter.

If you are flying through space in a straight line it is impossible to tell whether you really are moving or whether you’re actually staying still and the entire universe is flying toward you.

The same logic does not quite apply to spinning objects; the rotation of an object in space is directly observable because of the centrifugal (or is it centripetal) forces; to imagine that the object were staying still and the entire universe revolving around it* would require that we first explain the above forces and then explain what forces are causing the objects in the universe to move in curved paths around us.

*Sound familiar anyone?

So, can I do experiments here on earth that would determine that I am in fact on a spinning ball (assume I’m in a locked room with no knowledge of what lies outside)? This doesn’t even take into account the earth circling the and the sun circling the galaxy.

Yes

Yes. Foucault’s Pendulum for example:

http://www.si.edu/resource/faq/nmah/pendulum.htm

"The Foucault Pendulum is named for the French physicist Jean Foucault (pronounced "Foo-koh), who first used it in 1851 to demonstrate the rotation of the earth. It was the first satisfactory demonstration of the earth’s rotation using laboratory apparatus rather than astronomical observations. "

It depends on your reference frame. If you’re on the space station, then you’re probably interested in the centrifugal force: That’s the force pulling you down against the ground, and which is acting like gravity. Note that centrifugal force only exists in the rotating frame; it does not exist in an inertial frame.

And it’s perfectly reasonable to call the centrifugal force “artificial gravity”. It’s artificially produced, of course, and locally, it acts just like gravity would. Over larger scales, it doesn’t act the same as the gravitational field of a planet (it falls off with height in a different way, and you’ve got Coriolis effects), but who said that artificial gravity had to look specifically like the gravituy of a planet?

I did a quick pen on notepaper experiment. I turn the pad around while moving the tip of the pen in the same direction.

My quick, barbaric experiment created a path that looked like a circle until it hit the 'ground again.

Of course, this may have no relation to reality but could the flight path of the ball as looks from the thrower be a circle? I know from someone on the outside that the path would look like a straight line.

I don’t think that you can quite get a circle, but you can certainly get a sort of loopy path from a baseball thrown on a space station. I think it would, in general, be sort of teardrop-shaped, with a rounded top and a pointy bottom. If you threw it straight up, it would hit the ground some distance away.

All of this from the frame of reference of the space station, of course.

Sort of, but not quite, on-topic:

James P. Hogan’s book, “Endgame Enigma” involved a group of prisoners on a Soviet space station. A couple of them were physicists, and their escape plan required that they devise experiments to determine the size and rotational velocity of the station from within their windowless confinement. It’s sort of a fun book, written (obviously) before the end of the USSR and long before Hogan showed himself to be a Velikovskian (sp?) wacko.

The path of the ball depends on the speed of the rotation and/or the speed of the throw. Imagine that you throw the ball so that it travels the diameter of the ring at the exact speed that it will take you to go halfway around the ring. You throw the ball, it moves away from you and loops up, at one quarter of the way around the loop the ball is exactly overhead again. Then is starts falling, and eventually ends up exactly overhead again right before you catch it on the other side of the ring.

I wish I could draw the picture, or better yet an animation. I think you can describe many unintuative paths depending your speed relative to the rotation and the throw.

If I throw the baseball, i’m moving with a certain velocity right? then the baseball is moving with the same velocity. Does this velocity affects the travelling of the baseball? I think that the baseball will not travel in a straight line at the angle it was thrown, because i f there is this artificial gravity effect, it should probably work on all objets inside the spaceship. Of course, the greater the distance from the middle, the more effect there is.

Nope, nothing (except air currents) are opperating on the the baseball once it leaves your hand. The ball is completely uneffected by the rotating ring, as the is no gravity, just other forces that simulate gravity. You were accelerated up to speed along the ring by friction through your feet, the ball doesn’t have that.

Newton’s Laws state that any body in motion will remain in motion unless acted upon. What force acts on the ball once it leaves your hand? Nothing. The artificial gravity affect doesn’t work on all objects inside the space ship, only those in contact with the ring.

When you drop an object that you are holding it will appear to fall. This is because the object is travelling at a tangent to the rotation of the ring. It continues in the at path, which appears to you on the inside as if it is falling towards the ring. In actuality, it is travelling in a straight line, the tangent, and the ring is moving around so the same spot is underneath the moving object.

Consider an open ring, not closed. An object is thrown from outside the ring and passes through the opening. Should it all of a sudden drop to the edge of the ring because it happens to pass through? No, there are no forces from the ring acting on the object unless it comes in physical contact.

BTW - I’ve probably made a few errors, I last took physics 15 years ago.

You are right, Telemark, in your outside observer perspective. On the inside of the ring the Einstein’s EP demands that if we have no perspective we cannot determine from whence the acceleration cometh… ie any given acceleration is no different from another. We have a funky gradient in a ring ship, but in all honesty, with the blinders on, no thing can help us decide how the acceleration is occurring.

Same thing when you are in an elevator in outer space that is accelerated upwards at 1 g or on planet Earth in a similar box. No way to know whether you are on a planet with gravity or in an accelerating box. Gravity IS the acceleration we experience.