DescentBB

Imagine you’re in space, you’re not wearing anything you can take off, and you cant use any orifice as a thruster. Also you're not near any object. Could you flail your body around to start moving in a direction or turn?

you couldn't move in any direction. Newton says for every action there is an equal and opposite reaction. Another way to think of it is you can't create net momentum. So the only way to get moving in any direction is to make something else move in another direction.

The same is true for creating angular momentum. You can't do it unless you have some mass to expel somehow.

You could spin yourself. That's about it.

say if you did a fast 'sit up' and stopped. wouldn't that cause some kind of movement?

Isaac wrote:Imagine you’re in space, you’re not wearing anything you can take off, and you cant use any orifice as a thruster. Also you're not near any object. Could you flail your body around to start moving in a direction or turn?

No to direction...Yes to a turn.

Newtons Law applies here in that a body at rest will remain at rest unless acted upon by a force. In the case you presented for direction, you have nothing to push against so the answer is no. You will never be able to move away from where you are so wailing your arms or hitting yourself will not work.

However,

You can twist your body around its center of gravity in a way that changes the direction you were facing so in a sense you have moved.... but not in any direction.

Bettina

EDIT....wow all the answers came at the same time

No, you would not be able to spin.

You would not able to twist your body to face a different direction either.

but if there was air around you could swim in it so at least then you could move around right?

no

Let's forget the physics lessons and just talk common sense.

NO: you can not move yourself in a vacuum. You need an \"equal and opposiute reaction\". So the only way youy could move is to throw something: you would then move in the opposite direction.

YES: you *CAN* spin yourself. All you have to do is begin moving your arms in counter rotating directions in circles. This will rotate your body. It might not be in the direction you want, and your space suit won't really let you whirl your arms fast enough to create any centripetal force (The force which makes gyroscopes remain pointing in the same direction, and wheels hard to turn, and motorcycles stable at speed) but the principle is sound. It might take you some time to learn how to control your orientation, but you can do it.

As to \"swimming\" in zero gravity with air around you - yup that'll work. You wouldn't like it though, and you'd need to be VERY well co-ordinated, because you have to create more drag on the back stroke than you do on the forward stroke. This is trickier than it sounds, you aren't just lifting your arms out of water to reduce drag.

Ideally you'd have some hand-held paddles, which you'd be able to rotate 90 degrees for the forward stroke.

As to being in space without a space suit - well, you're fucked, so it doesn't make a lot of difference.

Exposure to hard vacuum will render you unconscious in 12 seconds or less, and you die fairly quickly from then on, by asphixiation. The whole \"explosive decompression\" thing is a complete load of bull★■◆●: Your skin is well tough enough to stop you exploding. Your eyes won't pop-out either. You might, however, fart quite a bit in the first few seconds, and you could suffer a bleeding nose as your sinuses are damaged by the lack of pressure. This will; be the least of your worries though, because you'll be in quite a bit of pain (quite a bit indeed) due to the unfiltered solar radiation, which will be blistering exposed skin even before you pass out. You may also suffer from the classic divers' complaint if you try to retain air in your lungs: your lungs will literally allow air to seep into your upper torso, and from there it migrates into the neck - expanding it dramatically. The resulting flesh feels \"like poking scrunched up newspaper\". Either way: make sure you exhale and leave your mouth completely open until all the air is gone.

If you want to see an almost perfect representation of what it's like - the scene in 2001 where Dave comes back into the Discovery through the emergency air lock, without his helmet, is as close as it gets to reality. The time he spends exposed is on the upper limits of how long people can remain conscious. But, as long as you managed to hit the \"CYCLE\" button before you passed out, you'd wake up fine, albeit with a real nasty headache.

No, you can't swim in air. It's not dense enough, and easily compressible.

And spinning your arms around won't do anything.

you can fan ur self. so i would work.

youre right, if you have a jet engine then you can propel yourself through air

If a bat can \"swim in air\" despite gravity, I don't see why a human couldn't move in air without gravity if he had a wing analogue (like Mobius's paddles). Not well or quickly, but he should be able to produce some movement. Treating the OP as a physics question and not a practical one.

ccb056 wrote:No, you would not be able to spin.

You would not able to twist your body to face a different direction either.

Ummm... Your wrong. Sorry.

Bee

Ok it depends if you're in a vacuum or in air.
In a vacuum i don't think you could start a spin. I'll have to try a test in 3d studio max to see what would happen in the physics simulator.

You could change your orientation just fine, just once you stopped flailing your limbs around you'd stop spinning and remain facing that direction.

I once saw a video of Skylab astronauts trying this very experiment. One of them carefully placed his colleague in the center of a large-enough room and that colleague then tried to move and turn. He was easily able to turn by spreading, twisting, and then closing his legs. He couldn't move himself from the center of the room, though.

Terminology note: momentum = mass * velocity; force = mass * acceleration. Velocity is the accumulated total (or integral) of acceleration, and momentum is the accumulated total (or integral) of force.

Krags wrote:You could change your orientation just fine, just once you stopped flailing your limbs around you'd stop spinning and remain facing that direction.

Exactly -- you couldn't create angular momentum. You can change your orientation by twirling your arms or legs such that they had angular momentum, which would give the core of your body an equal and opposite amount of angular momentum the other way... but the TOTAL angular momentum is still zero. As soon as you stop your arms, the rest of you stops as well.

Now, if we're changing the problem such that we're suspended in some sort of fluid, then you can move, spin, etc. All you have to do is give force/momentum to the fluid, giving yourself equal and opposite force/momentum. This is all just Newton's Third Law.

When a bird flies, its wings push air down and back. (When I worked for the Museum of Flight, I watched one of our brilliant former NASA scientists teach this to a bunch of five-year-olds using one of those rubberband-powered toy birds -- "do you feel the air blowing off of that? Some of it is coming back toward [kid A] and some of it is coming down toward [kid B], which is why the bird goes forward and stays up." I later adapted it to be a part of my Flying Gizmo show.) As long as we could create a mechanism for pushing whatever fluid we're imagining being suspended in back more than forward, we could "swim" through it, though perhaps not very quickly.

Right, you couldn't permanently change your original orientation.

ccb056 wrote:Right, you couldn't permanently change your original orientation.

Yes you can.

Bee

Granted it would be very difficult to survive long enough to do this but swinging your arms around in a circular motion would produce force that would rotate you around and basically provide you some attitude control. You just have to survive long enough and be able to produce enough angular momentum to create a centrifugal force.

http://en.wikipedia.org/wiki/Control_moment_gyroscope

RC

Only works if there's a gravity field to \"push off\" from around.

Bet51987 wrote:

ccb056 wrote:Right, you couldn't permanently change your original orientation.

Yes you can.

Bee

She (and everyone else who said so) is absolutely right.

Maybe you're thinking of movement like "bending your knees" and then "straightening". In that case, you'll end up exactly where you started.

But as RC mentioned, you could use movements like swinging your arms around to generate a turn, then opposite motion to slow/stop the turn at whatever point you needed.

If you were naked in a vacuum and were somehow able to survive, you would not be able to create any momentum at all. you could twist around to face any direction you please, but you cant move from point A to B, and you cant flail to make yourself spin constantly. the only way it would be possible is if you were to spit, or urinate. Lol.

if there was air, of course you could move. it would just be much slower than swimming in water.

As a side fact, if you put a 45kg ball of mass a meter away from a 90kg ball of mass, perfectly motionless to start out with, gravity would pull them together in about 4 hours.

Spaceboy wrote:If you were naked in a vacuum .... if you were to spit, or urinate.

I doubt you would be able to spit unless you still had air in your lungs to begin with. You might be able to slobber a little.

RC

Foil wrote:you could use movements like swinging your arms around to generate a turn, then opposite motion to slow/stop the turn at whatever point you needed.

Incorrect.

You could swing your arms to generate a turn, but as soon as your arms stopped, so would the turn. You wouldn't need to use "opposite motion".

Your arms moving already ARE the opposite motion from your body. You start stationary with your arms stationary, so the net angular momentum is zero. When you start spinning your arms, your body will counterrotate in order to maintain zero net angular momentum. And when you stop moving your arms, your body will stop leaving you with a total of zero angular momentum again.

Isaac wrote:Imagine you’re in space, you’re not wearing anything you can take off, and you cant use any orifice as a thruster. Also you're not near any object. Could you flail your body around to start moving in a direction or turn?

As people have said: you can rotate to face any direction (but the only way to give yourself a constant SPIN is to be constantly moving, once you stop, you stop spinning), but not move.
unless you have something like this...
here's some technologys that can get you moving in such a predicament:

If you're in open space, you can use solar wind (protons iirc), or the sun's emitted photons. With a "sail", you can then theoretically move anywhere away from the sun. If you are orbiting around a planet/moon or something then you can go anywhere you want because the sun will always be changing direction - just put your sail up during some times, and down during other times. I doubt it'd be too hard to equip a spacesuit with solar sails, sounds pretty cool actually.

OR, you might be able to harvest some reaction mass from space - Dust etc. Perhaps via electrostatics, i dunno. Then once you have some of this dust you can fire it away from you with some kindof particle accelerator. Hey, or you can just throw it.

Both techniques would take a while to build up momentum, but at least you'd be moving. It'd take you weeks, months, years to get anywhere.

The other (albeit rather inefficient) option is to take whatever loose crap happens to be Velcro-ed to your space suit and shove it in the opposite direction of where you want to go.

roid, why couldn't you sail towards the sun?

you can sail into the wind on a boat ...

ccb056 wrote:roid, why couldn't you sail towards the sun?

you can sail into the wind on a boat ...

Well, you can't really sail in to the wind but you can tack in it's direction so I guess if you have a lot of time on your hands (and what else would you do if you're just floating in space) you could probably do it.

From how i understand sailing into the wind ON WATER: i don't think you can do that in space, coz on water you press against the water to do that - and in space there's nothing like that to press against (so no need for a rudder). Unless you want to bounce off planets/asteroids or even nebula, maybe that could work. Point being i think it only works if you have some mass to \"swim\" through.

Maybe you could do it while approaching relativistic speeds though - pressing off the fabric of spacetime itself?

roid wrote:on water you press against the water to do that...Point being i think it only works if you have some mass to "swim" through.

Exactly. Or to put it another way: you can only sail at the interface between two mediums, and these mediums need to be in motion relative to each other. Sailboats "dip into" each medium.