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# Problems with 'artificial gravity' - centripetal force

Hey experts!

My problem is this:

I'm looking into generation of artificial gravity in a micro-gravity environment (space) but there are a couple of things I needed answered

a) What 'gravity gradient' can the human body tolerate (i.e. the difference in force applied to their head and force applied to their feet)
b) I've heard that high rotation speeds can cause sickness due to the inner ear - why is this? How does your body even know it is spinning without a frame of reference?

bc :)
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bitter_chicken
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1 Solution

Commented:
b)

when you are spinning very fast , wont the blood in your body travel to one end , due to the centrifugal force ? so the body will know that you are spinning ...

/abhijit/
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Commented:
Regarding b)  --  in outer space

To get your body spinning you have to give it some acceleration (e.g. pushing down with one hand a very massy shuttle).
What happens is that all small parts of your body have to cope with this acceleration, as they "try to stay" where they were before the move.
They finally (hopefully) are carried away with the rest, due to body material resistance.
This acceleration phase will be felt by the body.

However, unlike on Earth, once the rotation acceleration is back to null (e.g. still spinning but at the same speed), and after a while (a few minutes) to let your body come back to a stable state to cope with previous acceleration, you body should not be able to make the difference between its current state and the state it was before it was starting spinning.
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Author Commented:
In answer to avizit - yes; the imaginary centrifugal force (its actually our inertia) forces the blood to one end of your body - but that would not necessarily tell the body that you were spinning; the same effect is achieved on earth due to gravity! So you would feel as you do on earth - so how can you tell?

In answer to Mercantilum - a pointed answer... and intuitively I'd agree with you - but NASA's holdup in simply spinning to capsules at the end of a cable is the 'sickness' that astronaughts experience when spun at high velocity - even at constant 'speed' (constant magnitude of velocity). Apparently this can be overcome by the use of a very large rotational radius. I have part b) from a reputable source; I'm just trying to establish the reasoning....

bc :)

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Commented:
If we are talking about orbiting astronauts, they are still subject to Earth gravity.
It shouldn't - intuitively - be the case in outer space.
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Author Commented:
hmmm... thats a good point; there would be linear acceleration in orbit wouldnt there?

I'll get back to you soon....
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Commented:
You can make yourself dizzy by spinning around on earth.
The fluid in your inner ear sloshes around, as you spin, especially if you tilt your head at the same time.
The same think happens in space.
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Author Commented:
can you expand a bit on that ozo?
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Commented:
Already Galileo discovered that an object in motion not subjected to any external force continues its movement in a straight line.

When we are on a rotating platform (e.g. a car making a turn) we would like to continue our motion on a straight line but the car door excercises a side-force on us and keeps us in.  This is what we call centrifugal force, because it feels as if we were pulled outward (while in fact we are pushed inward).

But on rotating platforms there is also another "inertial" force, discovered by Coriolis almost two centuries ago, which only makes itself felt when we move with respect to the platform.  This force is proportional to the angular velocity of rotation measured as the angle sweeped in a second.  It also exists on Earth but is very weak, because the angular velocity is very low (360º/day). It is the Coriolis force which makes the hurricanes always rotate anticlockwise (and the cyclons in the southern hemisphere always clockwise).  We personally don't feel the Earth's Coriolis force precisely because it is so weak.

A space station is a comparatively small object, and to generate a significant gravity it has to rotate fast enough to make the Coriolis force noticeable.  This causes a "twisting" feeling with most movements we make.  Our inner ear is very sensitive and reacts badly to this force, causing nausea.

To understand the Coriolis force, imagine to stand on a rotating platform, like a merry-go-round but without horses and elephants.  The platform is solid.  Therefore, each part of it rotates with the same angular velocity.  That is, a floor plank close to the edge takes the same time in making a turn as a plank close to the centre.

Note that during a rotation the plank at the edge has to cover a distance equal to 2*PI*R, where R is the radius of the platform, while the plank close to the centre only covers the distance 2*PI*r, where "r" is much smaller than "R".  This means that the instantaneous velocity of an object at the periphery is higher than that of one close to the centre.

If you are standing at the edge and move toward the centre, your velocity "sidewise" must therefore be reduced (if you want to keep your feet on the platform!).  The platform does this for you by what you feel as a force applied to the soles of your shoes, exactly like the door of a car pushes your shoulder with the centrifugal force when you take a turn.  This sidewise push on your soles is the Coriolis force.

When you are a passenger on a car and it takes a sharp turn, move your head quickly.  You should be able to feel the funny twisting sensation due to Coriolis.
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Author Commented:
thanks for the extra info pointyears - get back to you soon
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Commented:

That dizzyness or "sea sickness"  that you describe is something that people can get use to over time.    Think of ballets-on-ice.  Some of those ice performers spin around for what seems like ages, but are then able to continue skating.  If I did that (assuming I could keep my balance while spinning) I would surely fall over immediately after stopping.

I think that any time you change rotation or gravity drastically your body will have to get use to it.  Even a concept as simple as landing on Mars, actually staying there with a different gravity and different rotational speed is really going to cause problems for people until their body gets used to their environment.  hell, even just changing time zones is problematic for some people and that doesn't involve gravity at all.

I guess what I'm trying to say is that the dizziness and what not might just be a reaction to a new environment and might just be a non-issue.   The body will adapt over time.
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Commented:
I agree with TAD that most people will adapt to the situation.  In fact, I don't remember where I read that most astronauts and military jet pilots get sick the first time they fly, but later they have no problem.

Concerning the gradient/difference of gravity between head and feet, if we assume the radius of a space station to be 5 times the distance between head and feet, the apparent gravity at the head level vould be 25% less than the apparent gravity at the feet level.

I have no idea how sensitive we would be to such a difference, but I would be surprised if it caused serious problems.  Obviously, a radius of 10 metres or more would reduce the effect to 10% or less.  Certainly harmless.
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Commented:
Rather than engineering a way out of the balance problem by having a large radius to the centrifuge how about a surgical solution. Drain the endolymph fluid from out of the cemicircular canals and insert valves to stop them filling up from the resevoir again in a proceedure similar to that for correcting Meniere's disease. No sense of balance is better than a sense of balance that works against you.
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Author Commented:
good thinking!
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Commented:
Perhaps in an abstract way.  I don't know you folks, but I would rather prefer to put up with some adjustment problem than to undergo surgery!

In any case, astronauts/cosmonauts have already spent months in zero-gravity and have been able to readjust to normal gravity with just some recoverable loss of bone tissue.  Therefore, if there is a need for artificial gravity, we can assume that it is because people are intended to spend many months or even years in the vehicle.

For long staying the structure would have to be quite substantial.  Therefore, even assuming that a head-feet gravity gradient of 20% would cause discomfort (which is not said), it shouldn't be a problem to have a 10-metre radius or more.
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Commented:

So far you've been focusing on how an astronaut will "feel" (equilibrium, balance, etc).  But what about some of the other things?

How does gravity affect the curvature of the lens over the eyes?

This is well beyond my field of expertise, but is it possible that the different gradients of gravity will have a visual affect similar to looking through water.
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Commented:
I have been considering the effects on balance and equilibrium because our inner ear is much more sensitive to movements and accelerations than anything else in our body.  That is why we all know motion sickness.

If other parts/organs of of our bodies were as sensitive, we would experience other effects just by driving a car.  I am not saying that there are no other perhaps even measurable effects, but even if there were, I expect that we wouldn't be affected by them.
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Author Commented:
I agree with you on that pointyears - the curvature of the lens is controlled by muscle contractions; not by gravity.
Trivia:
Looking through water causes distortion because the refractive index of the water is too similar to the refractive index of the lens of the eye, so the light cannot be refracted, thus it cant be focused. Thus little relevance to acceleration, i.e. 'perceived gravity'

bc :-)
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Author Commented:
Hey! I'm an expert! Up goes the point value!

bc :-)
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Commented:
So, the body was actually not in space, but orbiting :)
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Author Commented:
Good work pointyears, and thanks to everyone else for their comments...

Also, sorry no points to andyalder for his surgical solution, but the answer was a bit askew of the question - interesting nevertheless.

thanks for the great response!

bc :-)
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