# Qualitative differences between gravity and a spinning habitat

I'm aware that living in a spinning habitat has a couple of noticeable differences from being on terra firma. For instance, running in the direction of the spin ought to make you feel heavier, while running opposite to the spin ought to make you feel lighter. I have also read that a smaller, rapidly-rotating habitat is much more likely to make you feel sick.

But apart from these examples I struggle to imagine how day to day life would be noticeably different in a spinning habitat vs being on the ground.

What are some examples of day to day activities/phenomena/physical quirks that would be noticeably different while in a rotating habitat?

• doing this (whatever it's called) youtu.be/7TjOy56-x8Q?t=47 – uhoh Feb 12 at 1:22
• Basically throwing anything. – Organic Marble Feb 12 at 1:26
• ...Beer pong... – BobT Feb 12 at 3:13
• I might update my question to exclude activities performed in a fraternity party... – Ingolifs Feb 12 at 3:14
• @uhoh - judging by that video, every moment you make would be sightly nauseating – Mazura Feb 12 at 20:44

Depending on how close you are to the habitat's center of rotation, you'll feel anything from zero-g to the maximum the habitat is designed to offer - so while homes may be located in a one-gee zone, new opportunities arise closer to the spin axis. Zero-g manufacturing (for example: producing large crystals not possible in a gravity field; spinning extremely pure fiber-optic cable; assembling large structures that would immediately collapse if ever subjected to gravity, and so on) would be a distinct possibility, as would an interesting swimming pool. It would be possible to 'hover' over a particular point on the habitat with, say, a light aircraft one could pedal in the appropriate direction. So the people living in such a habitat will have ready access to about any gravity level they desire, compared to those of us living on a planetary surface. For older people who find gravity weighs heavily on them, they could move to a home closer to spin and maintain mobility in a way not possible here on Earth.

New types of sports will probably be invented, and as others have mentioned, those we already play would be affected thanks to the Coriolis effect (you can view a pretty good representation here. You mentioned running spinward/antispinward: those effects are lessened the greater the diameter of the habitat. If you took something the size of Island Three (up to 8 km in diameter), rotating to give its inhabitants 1G, you'd find your apparent weight would only change by little more than a pound. Hardly noticeable. If you'd like to play around with various radii, angular velocity, and so on, SpinCalc is excellent. I found a blog showing amusing effects on a ball's trajectory depending on how one throws it here, also worth reading.

• That blog link is fascinating, and well worth reading. Even with a general idea of the coriolis effect in your mind, there are still some surprising trajectories that occur. – Ingolifs Feb 14 at 1:42

First, I'll adopt terminology from Ringworld: "spinward" is in the direction of spin, and "antispinward" is opposite the direction of spin. And I'll say a bit about the Coriolis equation, but then go into qualitative effects.

Basically, anything that involves "up", "down", spinward or antispinward motion (which captures the fraternity-party activities in the comments), and a few other things, will be different from a "normal" gravity environment. The Coriolis equation $$\mathbf a_c=(-2 \mathbf \Omega \times \mathbf v) - \mathbf \Omega \times (\mathbf \Omega \times \mathbf r)$$ is opaque to most people who aren't scientists, engineers, or mathematicians, but it tells us what's going on.

The second term, $$- \mathbf \Omega \times (\mathbf \Omega \times \mathbf r)$$ is just the "artificial gravity" you feel. For a fixed rotation rate ($$\omega$$, the magnitude of the rotation vector $$\mathbf \Omega$$, so your "RPM") it varies only with your distance from the rotation axis, as long as $$\omega$$ takes into account any motion spinward or antispinward with respect to the station or spacecraft. The farther from the axis, the stronger the artificial acceleration, in direct proportion.

It's the first term, $$-2 \mathbf \Omega \times \mathbf v$$ that causes most of the trouble with our Earthly intuition about motion in our surroundings. It involves a vector cross product of the vectors $$\mathbf \Omega$$ and $$\mathbf v$$. $$\mathbf \Omega$$ is just the rotation rate $$\mathbf \omega$$ coupled to the direction the rotation axis points, and $$\mathbf v$$ is just the velocity vector as measured in the rotating reference frame. That cross product causes accelerations in a direction perpendicular to the velocity (and also perpendicular to the rotation axis), most un-intuitive if you've just come up from Earth for the first time.

OK, some observable effects.

You've already mentioned a couple: running spinward makes you feel heavier, running antispinward makes you feel lighter. If you could run antispinward at the same speed that your surroundings (hopefully, a tunnel!) are rotating, the first Coriolis term would exactly balance the second, and you'd be weightless. You could just float in the tunnel as the station spins around you—until a bulkhead comes up, then Whap! But it's tough to run that fast, for two reasons: the faster you run antispinward the less "weight" (artificial, of course) you have, so the less traction you have; and in stations or spacecraft of any decent, motion-sickness-preventing size, that speed is high. For a 100 m radius station, getting a measly lunar g (1.625 $$\frac{m}{s^2}$$) requires a rotation speed at the 100 m radius of 45.9 km/hr (28.5 MPH). Good luck getting up to that speed. And if you do—good luck with that bulkhead!

Motion away from the rotation axis ("downward") appears to induce antispinward forces, and "upward" motion sends things spinward.

An example: you're starting to take a shower. You know when the flow starts it will be very cold, so you don't stand directly beneath the showerhead, which is oriented to send water straight down. You turn on the flow slowly thinking you'd avoid sudden surprises. But you chose to stand antispinward of the showerhead, and to your great discomfort, the flow of frigid water that emerges from the showerhead straight downward curves antispinward, right onto you!

If, when you reacted, you jumped straight upward, you wouldn't land on the spot where you left, you'd land spinward of that spot. Out of the water flow, hopefully.

Motion parallel to the rotation axis doesn't get any such wierdness, until the downward acceleration of the artificial gravity induces an outward velocity, then the antispinward curve begins.

In my discussions with various folks contemplating the design of large rotating stations, the idea of sports on such stations has come up. Traditional sports such as basketball would be exercises in frustration as balls, shuttlecocks, etc., and you, move in ways you're simply not accustomed to.

• "Traditional sports such as basketball would be exercises in frustration". You mean an exercise in fun. I'd imagine people would soon get an intuitive sense of how to move and how other objects move. You'd quickly start seeing some very tricky shots exploiting the coriolis force. Sports fields would have to be arrayed in the transverse direction otherwise one team would have an advantage over the other. – Ingolifs Feb 12 at 8:07
• In sports we often compensate for directional biases on courts or playing fields by switching sides during a game. But in playing such sports, it would be critical for everyone to have a good sense of spinward/antispinward. I'm not sure about the magnitude of "soon"—whether it's days or months. But it's fun to think about! Hmm... mens' urinals might get tricky, though I expect the result wouldn't be any worse than I commonly see at airports. – Tom Spilker Feb 12 at 8:16
• "I'd imagine people would soon get an intuitive sense of how to move and how other objects move" Maybe, maybe not. Most professional athletes on Earth have spent 15+ years learning how balls work. That's a lot of unlearning to do, and it's in situations where semi-conscious split-second decision making and muscle memory often differentiate success from failure. I actually don't think we'd see any professional athletes successfully make the transition, but if we got to the point where people were born and raised on the station, that might start to get interesting. – GrandOpener Feb 12 at 21:03
• So... No window breaking baseball like in Interstellar? – JordiVilaplana Feb 13 at 16:43
• @JordiVilaplana Oh, you could probably still do that, just not exactly as they depicted in Interstellar. The ball's trajectory, as seen by someone standing on the cylinder's inner surface, would be curved. With the ball leaving the bat at the same velocity as in Interstellar you could still break a window, it would just be a different window. – Tom Spilker Feb 14 at 5:03

Below this answer there is a discussion about driving a car (generalizable to any vehicle) around the inside of a rotating cylinder. If you drove fast relative to the cylinder in the retrograde direction, you would experience less and less artificial gravity until you came to rest in the inertial frame, at which point you would find your self with no friction on the track or road. You may even float upwards away from the road with no way to get back down.

This could apply to bicycles or even runners as well, depending on the radius and rotation rate.