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Thread: Can you "swim" through air in zero gravity?

  1. #61
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    Thanks for replying, Jens.

    Although neither of us know quite what I did, I still would very
    much like you to describe what you imagined I was doing with
    my arms when you read my posts #38 and #46, and when you
    replied in post #48.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  2. #62
    Quote Originally Posted by Jeff Root View Post
    To do it I have to whip my arms around in front of me. To turn
    to the left I rotate my left arm clockwise from 12 o'clock and
    rotate my right arm counterclockwise from 12 o'clock.
    As I
    said, the centrifugal force on the blood in my arms seems to
    be a bit much for my vascular system to handle. My hands
    and arms are stinging again.
    Actually, I can't say I understand what you did, or really even understand the explanation you gave. What I'm picturing is, you jumped up into the air while holding both of your arms straight out in front of you. And after you jumped in the the air, you whipped your right hand so that it was touching your left shoulder, and your left hand so that it was touching your right shoulder, kind of like you were hugging someone. But I can't understand why that would make you spin in one direction or the other. . .
    As above, so below

  3. #63
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    If I may ask a question that might be tangential (hahaha... ok sorry.), what is the difference between angular momentum and "manipulate angular position"? I obviously don't understand this.

    Like Jeff, I attempted some acrobatics. I lined myself up under a ceiling beam extending left and right of me. I jumped up and touched it with my fingertips. The second time, after I lost contact with the beam, I twisted my hips 90 degrees and then rotated my upper body inline with my hips. This caused me to land facing to my left.

    I didn't have any angular momentum imparted by the jump, I wouldn't have been able to touch the beam if I had. Since I touched the beam, I *think* this would have killed off any tiny spin that I might have had on leaving the ground.

    I repeated this several times and found that if I passed 90 degrees, I started to tip sideways so that I land on only one foot. Were I brave/foolish enough, I suspect I could both turn 90 degrees and land flat on my side. Isn't this angular momentum at work?
    Solfe

    -----------------------------------------------------------------------------------
    "Triangles are my favorite shape
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  4. #64
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    Quote Originally Posted by Solfe View Post
    Like Jeff, I attempted some acrobatics. I lined myself up under a ceiling beam extending left and right of me. I jumped up and touched it with my fingertips. The second time, after I lost contact with the beam, I twisted my hips 90 degrees and then rotated my upper body inline with my hips. This caused me to land facing to my left.
    And it was FOR SCIENCE! Just don't hurt yourself.

    I didn't have any angular momentum imparted by the jump, I wouldn't have been able to touch the beam if I had. Since I touched the beam, I *think* this would have killed off any tiny spin that I might have had on leaving the ground.
    You used the beam to change your motion. It is so automatic to push off of things or to use friction that it often isn't consciously obvious how big a difference that makes. From personal experience, it can take a bit of work to avoid unconsciously applying small forces. (Yes, I've tried some of my own "for science" acrobatics, but not recently.)

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." Abraham Lincoln

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  5. #65
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    Quote Originally Posted by Van Rijn View Post
    And it was FOR SCIENCE! Just don't hurt yourself.

    You used the beam to change your motion. It is so automatic to push off of things or to use friction that it often isn't consciously obvious how big a difference that makes. From personal experience, it can take a bit of work to avoid unconsciously applying small forces. (Yes, I've tried some of my own "for science" acrobatics, but not recently.)
    I am going to have to think of a better trick, I am still unconvinced that a person in free fall can't create a spinning motion.

    I think I will read the Darwin Awards, not for inspiration, but to put a damper on fool hardy experiments. Perhaps scale models would be better.
    Solfe

    -----------------------------------------------------------------------------------
    "Triangles are my favorite shape
    "Three points where two lines meet"
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  6. #66
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    Quote Originally Posted by Solfe View Post
    I am going to have to think of a better trick, I am still unconvinced that a person in free fall can't create a spinning motion.
    The key is in one of the phrases I quoted above: "Unlike the angular-momentum twists of most twisting somersaults, zero-angular-momentum twists persist only as long as the upper body is in motion in relation to the lower body. If the diver holds his body completely rigid, the twisting motion immediately stops." If your total angular momentum is zero, it will remain zero unless you experience an outside torque. However, that's your total angular momentum. Since you're not rigid, different parts of your body can have different amounts of angular momentum, as long as the total remains the same. You can move one part of your body, resulting in motion in another part of the body. So if you start twisting your hips or arms around while you're moving, yes, conservation of angular momentum will make the rest of your body twist accordingly. For example, if you start moving your arm around like a windmill, the rest of your body will start rotating the other way. However, you still haven't imparted any net angular momentum in the process. Your total angular momentum remains zero, and if you stop twisting around, you'll stop spinning. Conversely, if you were already spinning before you started, you may be able to change how you're spinning by contorting in the right ways, but when you stop moving around, you'll find that you're still spinning. The direction of your spin will still be the same as it was before, although your body's orientation relative to that angular momentum vector might be different. That is to say, suppose you were an astronaut floating in orbit with your feet pointed toward the Sun, rotating about the axis running from your feet to your head. You might be able to change your position so that the sun is now on your right side, or above your head, but you'll find that if you stop actively moving, you'll still be spinning on that same axis pointing toward the Sun.

    One of the reasons this is hard to experience is that the duration of free fall is so short. It's easy to twist a little bit by doing some gymnastics while you jump. However, you hit the ground pretty soon, and so it never becomes obvious that the twisting is a result of your active movement, and that you wouldn't just keep spinning once you managed to start it up if you were in free fall longer. The article Jeff posted really is a very good discussion of this if you haven't read it yet.
    Conserve energy. Commute with the Hamiltonian.

  7. #67
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    I'll get a camera and make a video. Until then...

    A slightly more detailed description of what I do:

    I start with my feet shoulder-width apart, and bend at the knees
    so that I can jump up by straightening them quickly. I also bend
    forward slightly. I'm not sure why, but it helps. I put my arms
    down and back just far enough that I could put them behind my
    butt, so that I can use them to give me more height. My upper
    arms are at about a 45-degree angle to the ground. The first
    motion is to swing my arms forward and slightly down. About
    the time my hands are at the bottom of the arc -- just after they
    have passed by my sides -- I start to straighten my legs and my
    back. I also start bending my arms at the elbows so that my
    hands will swing in a shallow arc rather than going far out in
    front of me. By the time my upper arms are at a 45-degree
    in the opposite direction from where they started (toward the
    front instead of toward the back), my arms are bent 90 degrees
    at the elbows, so that my "lower" arms are at a 45-degree angle
    upward, and my hands are at the level of my face. At about
    that time I also start rising on my toes.

    I think my legs go straight at the knees at the same time as my
    toes leave the floor. My upper arms are now at a slight upward
    angle -- less than 45 degrees -- and that is as high as they get.
    My elbows are at the level of my eyes, and my "lower" arms are
    straight up-and-down. My hands are still to the sides, but well
    above the level of the top of my head. I could raise them much
    higher yet, to touch the ceiling, but normally don't.

    At this time I'm approaching apogee. My body is vertical and
    my hands are still above the top of my head. I lower my arms
    and swing them to one side. If I want to turn to the left I swing
    my arms to the right. My left elbow goes in front of my left
    nipple (never referred to *that* online before!) and my right
    elbow follows a path that appears to be pretty much straight
    down, relative to the ground. It ends up slightly behind me
    when I touch the floor again. My left "lower" arm ends up
    roughly horizontal or a bit lower, with my left hand near the
    right side of my chest at the level of the lowest ribs. My right
    "lower" arm rapidly goes from straight up to straight down,
    directly below my elbow, and a little bit behind me. So that
    would explain why my right arm and hand feels the brunt of
    the force, since I have mostly been turning to the left. My
    right arm and hand are generally a couple of inches away
    from my body when I touch down. At no time is either arm
    very far from or very close to my body. The distance does
    vary some, but not much.

    While I am doing those arm motions on the way down, I am
    twisting my body around in a way I don't understand. It does
    not feel very strenuous. It feels as if my entire body turns.
    By looking in a mirror I can see that my upper body is facing
    the same direction as my lower body at the instant I touch the
    floor again.

    Because of the effect on my hands, I have tried to limit how
    hard I swung them, and did not get turns of more than about
    120 degrees this session. My hands hurt again, though. As
    Jens said, I'm sacrificing them. For science, as Van Rijn said.

    Quote Originally Posted by Van Rijn View Post
    You used the beam to change your motion. It is so automatic
    to push off of things or to use friction that it often isn't consciously
    obvious how big a difference that makes.
    Solfe was apparently touching the beam only as a reference to
    ensure that he didn't start to turn until he was off the floor. He
    might as well have touched a bit of thread hanging from the
    ceiling for all the difference it would make. I tried touching the
    ceiling before turning. There is no way on or off Earth that a
    touch like that could impart any significant rotation. And why
    would it be in the desired direction rather than the opposite
    direction?

    -- Jeff, in Minneapolis

    .
    Last edited by Jeff Root; 2012-May-03 at 04:34 AM. Reason: missing letters
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

  8. #68
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    Quote Originally Posted by Jeff Root View Post
    Solfe was apparently touching the beam only as a reference to
    ensure that he didn't start to turn until he was off the floor. He
    might as well have touched a bit of thread hanging from the
    ceiling for all the difference it would make.
    He said he thought it affected his rotation, and my own experience is that it often isn't obvious how much effect these things can have, especially when combined with other actions.

    "The problem with quotes on the Internet is that it is hard to verify their authenticity." Abraham Lincoln

    I say there is an invisible elf in my backyard. How do you prove that I am wrong?

    The Leif Ericson Cruiser

  9. #69
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    I did basically the same thing Jeff did, and I think I have an answer, though I don't have the physics knowledge to back it up properly.

    When I jump up, I wait until I hit the top of the jump, just make sure I don't have any rotation from the floor. The act of swinging my arms hard to one side makes my body rotate in the other direction. This is the same thing you do in a swivel chair by swinging an arm to pivot slightly to one side. So far so good.

    The problem comes from the ground on landing. At the moment of touch down, my body is facing about 3 or 4 o'clock, but my hands are about 11. My friction on the ground is what stops me from coming back to a neutral position with each. In a swivel chair, is the friction on the bearing at the swivel point.

    I think the thing to look for are high divers who can jump, spin one direction, then stop that rotation and spin in another before making contact with the water. A simple jump just doesn't give enough time in the air.
    I'm Not Evil.
    An evil person would do the things that pop into my head.

  10. #70
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    have you not seen a video of a cat dropping upside down but landing on its paws? It rotates its central body relative to its legs to achieve the turn and then pulls its centre back in line on the ground. Cats have very flexible spines.

  11. #71
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    profloater,

    So let's assume that the "central" part of a cat has the same
    moment of inertia as that of its ends, which includes the legs.
    It starts out upside down, as in the photo sequence in the
    PDF I linked previously:

    http://www.physics.utoronto.ca/~phy1...d%20Twists.pdf

    The cat rotates its center 180 degrees to the right in order
    to make its ends rotate 180 degrees to the left, correct?
    So until it lands, the cat contains two 360-degree twists,
    correct?

    A very flexible spine indeed!

    Of course, the photos in the PDF show that the cat does
    nothing of the kind. But I can't say that I understand
    exactly what the cat actually does.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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