In the anime series Mobile Suit Gundam, one of the justifications of having giant humanoid robots in Space is the ability to use limb (and later on wing, tail) movements to adjust heading without using reaction thrusters. The real reason why they’re humanoid is because it looks cool, of course, but I’ve always wondered if in fact a human, can turn in space without using thrusters.
I believe that some satellites use flywheels and conservation of angular momentum to do this. But can a human, using only arms, legs, torso twists, etc, change direction? I had though that maybe you can extend your arms, throw them one way, in the process turning your torso slightly in the opposite direction, and then retract the arms and turning back. Would that just get you back to where you started, or could change the way you’re facing that way?
Alternatively, could you rotate your arms and turn VERY slowly, like using a flywheel? How difficult would it be to control the turn direction?
You can twist your body around, and so change orientation, but when you untwist, you’ll still face the same way. It’s not as if there’s anything in free space that will give resistance against your turning (That’s what’s keeping you from turning back if you go s-l-o-w-l-y, as you suggest – In water, there’s still some resistance.)
Of course, you could always arrange things so you’re not twisting your body, and you have adjusted something that you don’t mind not being turned back. Mount a weight on a rotating belt around your middle, for example.
I understand that cats “fall on their feet” by spinning their tails in the direction opposite to the direction they are turning their bodies. I saw a sequence of pictures once, with a scientist holding a cat upside down by the feet above a table, dropping the cat, and the cat twisting around to land on its feet.
So, it might be possible to rotate your body by flailing your arms. Or, you could just install a tail on your Giant Powered Armor.
No, the cat’s turning in midair is pretty close to turning in space – gravity neither helps nor hinders turning, and wind resistance is negligible.
Life magazine had pictures of this, back in the late 1960s.
Our cat Mignight landed on her feet pretty well, even though her tail was paralyzed, and useless as a counterweight. She must have done it all with twisting her torso.
Hmm, I did manage to find this Wiki article on cats. Is air resistance really negligible in these cases? Seeing as how they have a fairly low terminal velocity, wouldn’t the cats be able to use wind resistance to their advantage? Isn’t that how human skydivers move about?
Not necessarily. You can twist around in such a way that when you untwist, you’re facing a different way. You can’t impart any angular momentum to yourself in this way, nor can you change your momentum, but you can do whatever you want to your orientation.
For instance: Position yourself in zero-gravity (or if you don’t have easy access to zero-g, a swivel chair with good bearings) with your right hand touching your shoulder and your elbow pointed straight down. Thrust your right arm straight forward, so your arm is fully extended. Now keep your arm extended, but pivot it around so it’s sticking straight out to your side. Now retract your arm back so your elbow is pointed down and your hand is touching your shoulder again.
To start with, you have zero total angular momentum, and there are no external torques, so your total angular momentum will remain zero throughout the exercise. While you’re thrusting your arm out, if we approximate your upper arm as identical to your lower arm, your arm has no angular momentum, so your body has no angular momentum. While you’re swinging your arm to the side, your arm has angular momentum in the -z direction, so your body must have the same angular momentum in the +z direction (since your total is 0). While you’re retracting your arm, your arm and body again have zero angular momentum, just like when you were extending it. In total, then, your body has had some time with positive angular momentum, and no time with negative angular momentum, so your body has rotated some, even though your arm came back to the same position relative to your body.
Or just swing one arm around in circles above your head - your arm is rotating one way, so your body reacts by rotating slowly in the other. Angular momentum is conserved and when you stop doing it, you stop rotating - facing in a different direction to where you started.
Or, of course, extend both arms out sideways and describe circles with your fingertip - you will roll forward or backward depending on the direction of the circles you make.
I have a question about that Skylab video of the astronauts “running” around the ring of storage compartments. At most it is a few second so it’s hard to tell exactly what is going on, could those men actually run around that ring as if was rotating for artificial gravity. If they stopped I assume they would just float away but would they remain “glued” to the surface as long as they were moving, could you keep moving in that circle?
Yes, because your body wants to keep moving in a straight line, which means you keep on encountering the curved wall as you run. No different to the bikes on those wall of death things - except that those have the additional force of gravity at their side.
Yes, that is how you maneuver in freefall. You do have to be moving fast enough relative to the air (in order to deflect enough mass to generate the force needed) though. IME it took about 3 seconds of freefall from a standing start (such as a hot air balloon or a hovering helicopter). Exiting from a plane is different as you are already going about 100mph when you hop out the door.
If you aren’t moving fast enough you tend to flail about - you can certainly make some changes in position by twisting and turning but it’s nothing compared to the maneuverability that you have at full speed.
I don’t see it. When I thrust out my arm, I’m moving both the upper and lower arm in the same direction. It’s got a lot of angular momentum, and in order to keep my body’s total equal to zero, my body will rotate back the other way (I’ll also rotate backwards around the other axis, since my body’s center of mass is well below my should, but we’ll ignore that). When I swing my arm around, my body’ll rotate back the other way. I don’t see how I can change position with all the bod parts in the same relative position. I can see facing a different way if something has a different orientation relative to the rest.