Simple physics question, I think, with regard to swinging your arms to regain your balance

With your feet on the ground, if you begin to lose your balance and fall backward, you can swing your arms in a circle, front to back and regain you balance. Same for if you begin fall forward, you swing you arms forward. (To clarify the direction is defined by the top of your swing).

But you can’t do that in space. I realize that your momentum cannot be altered in space, but could you contort your body to face the other direction?

Could you get yourself to spin?

Sorry, I changed the op a couple of times. But I would imagine you could impart a spin upon yourself in space by spinning your arms then bringing them back in like a ice skater.

I know I’m probably wrong but why?

You can do that in space (and it’s done all the time using products that are just about as close to “commodity” status as space hardware gets), if I understand correctly what you’re asking about.

I’m not sure what effect you intend this to have, beyond that of a reaction wheel; could you clarify what you want to achieve? It seems to me that, theoretically, your body’s spin rate would increase if you could somehow pull your arms in while keeping them spinning just as fast relative to your body, but you have to spin your arms relative to your body first, and that will impart a spin to your body in the first place.

To keep your body spinning (rather than just changing orientation and then stopping), you have to keep your arms spinning the other way. This is possible if you’re spinning them in the same way as you do to keep from falling forward/backward on Earth, but not if you’re trying to spin about your vertical axis. Even spinning my arms continuously in the former manner, I don’t think I could pull them in and keep spinning them usefully.

Here are a couple of videos that might answer some related questions for you without you having to figure out how to ask them.

The first video shows an astronaut trying to escape a spot in the middle of the International Space Station’s Kibo module, which (before all of the equipment was installed) was a spot where no handholds/footholds were reachable. He demonstrates some moves you might find interesting.

The second video is from Skylab, showing some other weightless gymnastics:

Sure you can. In both cases (earth or space), this works because of conservation of angular momentum.

Most folks intuitively understand conservation of linear momentum: if you’re floating in the middle of the ISS, and you shove a bowling ball in one direction, your body will move in the other direction. The bowling ball and your body, considered as a single mass, have a fixed location in the ISS; that’s true before and after you shove the ball away.

Angular momentum can be thought of in an analogous way. If you’re floating in the middle of the ISS, all of your matter has a fixed total angular momentum. If you start moving your arms in a circle, you are giving them angular momentum in one direction of rotation, and the rest of your body will develop angular momentum in the opposite direction of rotation. Stop moving your arms in that circle, and the rest of your body will stop rotating in the opposite direction. The entire time, your arms and the rest of your body, considered as a single mass, will have the same total angular momentum as when you started.

Somewhere out there is a video of Skylab astronauts demonstrating a version of the above exercise. IIRC, there were two astronauts floating stationary side by side, both facing the camera as if standing at attention. On cue, the both start flailing their arms in a “stirring the cauldron” kind of motion, and do something similar with their legs. They both begin to rotate left on a head-to-toes axis; once they’ve turned 90 degrees to their left, they stop flailing, and their rotation stops. On cue, they start again, and stop when they’re facing away from the camera. A couple more cycles of this, and they’re facing the camera again. I’ve looked for this on YouTube, but can’t find it. Maybe someone else can?

Found it. Turns out it was space shuttle astronauts, seen at 2:15 in this video:

The video title claims these are Skylab astronauts, but the video definitely shows a mix of Skylab footage and space shuttle footage - and judging from their clothes and the cramped quarters, the clip at 2:15 is from the space shuttle.

Judging by the number of people visible, too. Skylab, being visited using Apollo vehicles, and only one of those at a time, only ever had three astronauts at a time.

The flipside of that is that, while you can change your orientation, you can’t change your net rotation. If you find yourself with some rotation that you don’t want, you can temporarily stabilize your torso by putting all of the angular momentum into your limbs, but as soon as you stop moving your limbs, your torso will start rotating again. Or if you want to be rotating but aren’t, you can only achieve torso rotation as long as you keep windmilling your limbs.

Conservation of angular momentum is a useful tool for motocross riders. When they are airborne during a jump, the motorcycle may begin to pitch toward nose-down or nose-up, depending on exactly how they launched from the ramp. For small jumps with short flights this isn’t usually an issue, but if they’re in the middle of a big jump that has them airborne for a few seconds, that change in pitch can become a problem on landing if it’s not addressed in mid-flight. Fortunately they have a solution:

  • If the front of the bike is pitching down, twist the throttle. The rear wheel accelerates so that the front of that wheel is moving downward; conservation of angular momentum results in the chassis rotating in a direction opposite to that rear wheel (or at least ceasing/slowing its nose-down rotation).

  • If the front of the bike is pitching up, step on the rear brake. Similar concept as twisting the throttle, but with the opposite effect: the nose of the bike will stop pitching up, or may even start pitching down.

Good video here:

Conservation of angular momentum is also the thing that makes the SawStop’s safety mechanism work. Just in case you haven’t heard of the SawStop, it’s a table saw with a mechanism that detects when the blade has made contact with your body; when that happens, it fires a braking mechanism to quickly stop the saw blade. The entire blade is mounted on a swingarm, so that when the rotation of the blade is stopped, the blade’s angular momentum is transferred to the swingarm, causing the suddenly stopped blade to rapidly retract below the surface of the table:

Could you also not use a combination of belches and farts as thrusters? You could pivot by turning with your head and pelvis. I’d be surprised if no astronaut hasn’t thought of this and experimented on his own as they are up there for long time and get bored.

Could probably have better results by pursing your lips and blowing with your lungs. “Maximum expiratory pressure” is on the order of ~2 PSI. So if you can purse your lips to a 0.25" orifice (0.05 square inches), you could maybe achieve 0.1 pounds of thrust, and sustain it for as long as you can exhale.

Matt Damon’s character in “The Martian” developed wild levels of thrust by piercing the glove of his spacesuit. But his suit would have been at 14.7 psi at most, and likely less than that (NASA astronauts on EVA use pure O2 at about 5 psi to allow better dexterity/mobility). A hole small enough to not catastrophically depressurize his suit would have only generated a fraction of a pound of thrust.

Cats do this if they fall in an initially bad orientation. They’ll twist their bodies to temporarily redistribute angular momentum between core and extremity, until they’re oriented the way they want to be.

The orientation they want is on all fours but with the front lower than the back, so their front paws land first. I’m not sure why this is. Cats falling from too high often damage their chins as a result.

You could do it just by inhaling, then blowing - a directional blown jet of air imparts reaction in a more defined direction than the inhalation, which is less directional.

If you attach a piece of buttered toast to the back of a cat, then throw the cat at the floor, it will never land, it will simply hover over the floor rotating because the both cat landing feet first and the toast hitting the floor buttered side down cannot happen at the same time.

I’ve been reading science fiction since the early '50s and it was not uncommon for a space ship to change it’s pointing direction via hand cranked flywheels, or as they are called now, reaction wheels.

If you’re riding a bicycle with no suspension, and you do a tall vertical drop, landing with both wheels at the same time hurts. The pedals and handlebars both come to a stop very suddenly, cushioned only by the compression of the tires; you have to arrest your own body’s momentum with your arms and legs, and the addition of an inch or two of compression from the tires is of modest help here. A better way to get through a really high vertical drop is to land with your front wheel very high. When the rear wheel hits, the chassis starts to rotate forward, which means the pedals are still moving downward (but more slowly), so you’ve got greater total distance/time through which your feet and legs can absorb momentum. At the same time you can resist arm extension to arrest some of the chassis’s angular momentum as it rotates forward. And then when the front wheel finally lands, you can resist arm flexion to absorb the rest of your body’s (now greatly reduced) momentum.

I wonder if maybe cats are doing something similar, i.e. finding a way to arrest their vertical momentum over a longer period of time by temporarily converting to angular momentum, thereby reducing peak impact forces. If so, it’s not clear why they would prefer a front-feet-first landing instead of a rear-feet-first landing.

Yes – this was the part I meant when I said “I’m not sure why this is.”

They’re odd in other ways sort of similar. They really don’t like to back up, and if you try to force them to, they may go up on their hind legs and fall over backwards. They ascend trees front-first, which is great because their claws work to hold them up, but they dislike going down their head up and will turn around and run forwards down the tree as high up as they think they can manage. Perhaps some of this is because, like most four legged mammals, they carry most of their weight on the front legs.

Yes. This is why spacecraft with reaction wheels usually have RCS thrusters still, which they use to remove angular momentum from the reaction wheels, because those do have a maximum speed they can spin at, and therefore a maximum angular momentum they can store. Generally, you want to keep them from getting close to that maximum, because, if a RW reaches its maximum, it’ll stop being able to properly control the spacecraft’s spin about that RW’s axis.

I’m currently working on a solar sail spacecraft design, and I’m trying to avoid any thrusters at all, so I’m considering desaturating the reaction wheels using light vanes like some of the Mariners used.

I think it was Isaac Asimov who had one of his characters get out of such a pickle by taking off his underwear and throwing it as hard as he could–the underwear was literally the only throwable thing he had.

Wow! Never heard of that before. Were those small or large ships? What were the ostensible advantages to hand-cranking them? (I guess: Humans, instead of computers, having very fine control of the spacecraft’s orientation, useful in situations where the humans know where they want to point. Usable if the power is out or the computer is down.) I realize computers weren’t as powerful then, but spacecraft did control their orientations autonomously not long after that. Maybe just to add interest to the story?

This also reminds me that I’d heard that control moment gyros (like reaction wheels, but changing the spin axis instead of the rotation rate) were invented in the 1990s (by some NASA center or other), but I just saw that Skylab (launched 1973) had them, so that can’t be right.

I read these “space operas” a long time ago. Might have been small ships run by “belters” and it was cheap, but worked.