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I'm not a smart guy, can't find an answer to a question:

Everything I see involves a large construct spinning at a slow speed to create gravity. Instead of something large could you not use a, let's say 2 ton, mass centered in a object and increase the speed to increase the gravity effects to an acceptable level? So a 2 ton weight sphere at 500rpm to generate the same field of gravity as a larger object at a slower spin?

Would that not eliminate the spin sickness that everyone talks about? and as the object increase in size you just spin the heavy object in the middle faster to increase the gravity.

It might be a dumb idea, if so sorry. It's just a weird stuff that pops in my head sometimes and I'm not a math guy so don't know if this would work or not.

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    $\begingroup$ You mean artificial gravity? $\endgroup$ – user6760 Nov 5 '17 at 6:19
  • $\begingroup$ The mass of the spinning thing doesn't matter. That is, the spinning does not somehow increase the mass's gravity. $\endgroup$ – Tony Ennis Nov 5 '17 at 14:34
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This is a matter of physics. Spinning structures to generate artificial gravity works because of centrifugal forces. The 'gravity' is caused by the centrifugal forces pushing everything down towards the floor. Where the floor is the inner surface of parts of the structure moving outwards from the centre.

The whole structure needs to move, to be spinning, to generate the centrifugal forces. In any part of the structure that isn't spinning, there will be no centrifugal forces and no artificial gravity. Therefore, if only a central mass is spinning, and the rest of the structure isn't, there won't be any artificial gravity in the non-spinning parts.

Centrifugal gravity isn't real gravity because it is the result of the rotational motion of the spinning structure. It would be really nice if we could simply generate artificial gravity by spinning large masses. Unfortunately, nature decides gravity wasn't going to be like that.

Spin sickness will not be eliminated because it still has to spin.

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TL;DR: no, things don't work that way.

To create the impression of gravity by spinning, you need to be inside the spinning object. Mass is nearly irrelevant to this. The characteristics that matter are spin speed and distance from the center.

If you are standing on the spinning object, the spin pushes you away from the object. This makes us slightly lighter on the Earth. But overall, the gravity effect (unrelated to spin) is greater than the push away from the surface. On a small, fast-spinning object, you could actually be pushed off by spin.

Once you lose contact with the spinning object or if you never were in contact with it, the spin becomes irrelevant. You now have a straight line velocity and an acceleration towards the mass. If the velocity is high enough (called escape velocity), you will eventually move far enough away that the acceleration by gravity becomes negligible.

You cannot escape spin effects by spinning an object that you are not touching to create a gravity-like effect. It won't do anything. You need to be spinning with the object for the gravity-like effect.

What's actually happening is that the spin gives you a straight line velocity tangential to the circle of the spin. But because there is a centripetal force, i.e. a force pushing you towards the center, your direction is constantly changing such that you follow a circle and feel like you are being pressed down. But you are actually traveling out and being pushed up.

You have a similar effect in a fast rising elevator. You feel like you are being pressed towards the floor. But what is actually happening is that the elevator is pushing you up. You are trying to stay where you are. This gives an impression of increased gravity on a fast elevator. But really, gravity is the same. It's just that inertia is joining it.

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    $\begingroup$ Concerning the elevator: This is of course only true while the elevator accelerates. $\endgroup$ – Michael Nov 5 '17 at 10:22
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Gravity as we know it is generated by mass: any object whose mass is different from zero creates a distortion in the space time, which we experience as gravity. Or, to put it in Newtonian terms, any object with mass attracts all other object with mass. The more the mass, the more the pull.

When we are in a rotating reference frame, we experience a fictitious force, called centrifugal force, which seems to push us outward: like a shirt in a wash machine drum during centrifuge. In certain condition this effect can be used as a substitute for gravity, but it is always due to the rotation, not to the mass.

This centrifugal force depends from the square of rotation rate and from the distance from the rotation center. As you see, no mass is involved.

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As noted in other objects, when you see in a work of science fiction (or in a science report about things we might build in the future) that a large object is rotated in order to create artificial gravity, the mass of that object has nothing to do with the "gravity" that is experienced, nor is the spinning creating any kind of gravitational field in or around the spinning object.

What creates the sensation of gravity, if you are in a room within the spinning object, is the fact that the room you are in is traveling along a curved path. The amount of gravity created is purely a function of the radius of curvature of the path the room is following and the speed at which the room travels along that path.

You don't need a massive structure with huge rings to get this effect. At least in principle you could build an airtight capsule just around the room you want to be in, attach a long cable (called a tether) to the capsule just "above" the center of the ceiling of your room, attach a suitable counterweight to the other end of the tether, and set the room and counterweight moving so that they sit at each end of the straightened tether and revolve around a point somewhere between them. As this structure traveled through space it would "spin" somewhat like a drum major's baton when it is thrown in the air, but much slower.

Such a scheme is discussed here and here. The tether needs to be quite long and there are various engineering challenges to overcome in practice, but if you could overcome them, you could have significant artificial gravity aboard a structure of this kind without causing its occupants to have "spin sickness." The structure would still be "large" if you just look at its overall length, but it would have very little mass and volume, even including the tether and counterweight.

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Short answer: No, because The sensation of gravity in a rotating object isn't caused by the mass of the object. It's caused by the rotation of the rotation.

Longer answer: On the scale you are discussing (small to modest space station) the object's mass will generate only a tiny amount of gravitational attraction. To understand where the rest comes from (which is the great majority of the "gravitational force" sensation) needs some discussion of acceleration and forces.

Simplifying a lot, and in what physicists call an inertial frame (broadly, the entire area of space you and the space station are in, isn't accelerating or changing, and can be approximated as just moving at a constant velocity), this is what's going on:

  • Newton's Laws of Motion state that acceleration is proportional to force. If force is applied to an object that is completely free to move, it will accelerate. The reverse is true as well: if an object is accelerating, then some overall force is being applied to it.

    (Some exceptions are things that aren't free to move, which means another force is opposing the first force, or things that deform or where the energy doesn't go into accelerating it.) For example, we don't accelerate to the centre of the earth, because the earth's surface pushes us up, just as much as gravity pulls us down.)

  • The catch is that acceleration isn't just "in a straight line". To move at the same speed along a curve, is also acceleration, because velocity (which includes both your speed and your direction) is changing. Your speed might be the same, but your direction is changing.

    So if you are moving at a constant speed in a circle, you are, in fact, accelerating, because your speed might be always the same, but your direction is constantly changed.

  • If you are accelerating, then there must be a force pushing or pulling you, to make you accelerate. In a rotating space station it's the outer walls of the space station. As you stand on them, they are constantly applying a force to you (which you can tell by simply noticing you're moving in a circle!)

  • It is this force, or rather the sense that you are constantly being pulled/pushed by a force, or resisting one, which the person in the rotating space station feels. Your body "wants" to move in a straight line, it's being accelerated by forcing it to move in a curved line, the outcome is it feels a force is being applied to it.

  • A force is a force - the force that gravity applies and the force the outer wall applies to your body, are both experienced much the same (assuming it's large enough not to feel it as circular motion, or you are acclimatised).

  • The space station's rotation will be calculated to provide the desired force to the people inside it. Depending on its size and rotational velocity any force could be obtained from very low to very high).

Therefore you can see that it doesn't matter how heavy the space station is. What matters is its size and rotation rate. On this scale, that is all that determines how many "g"s you feel.

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