I was just reading a SF book about a theoretical material that was completely frictionless. The author wrote that if a person was dropped out in the middle of a piece of this material, he’d be trapped - even if the surface was only a few meters across - because there would be no way to move off of it unless you had a rope or something to throw outside of the surface and use to pull yourself out.
This sounds wrong to me. I was thinking if you simply fell forward you would give yourself some momentum in that direction. Or if you threw something away, you’d give yourself some momentum in the opposite direction. Or if you just made swimming motions against the air resistance. As long as you could achieve some minimal movement, you’d be able to eventually glide off the surface.
I suppose you’d have to be moving enough to overcome the friction of the air but if you want you can assume it’s a vacuum and you’re wearing a spacesuit.
So am I right and the author wrong? Or am I wrong and you would be trapped?
I don’t believe falling forward (or backward) would work. Conservation of momentum would say your center of mass would not move. Throwing something away should work perfectly. You’d slide in the opposite direction until the air stopped you. Swimming motions in the air would also work provided you were careful to feather your hands when bringing them back to the front to make sure didn’t undo the effect.
I think you are right. Falling forward isn’t likely to work, your feet would slip out from under you right away, instead of giving you forward motion. However, air has more than enough resistance to allow you to swim your way to the edge, eventually. I would use a breast stroke type motion. You could also take off a shoe and throw it, giving you momentum the opposite direction.
OldGuy is right about falling over not working; your center of mass would move straight down, since that’s the only direction that a force can act on you. In practice, this means that your feet would slide out from under you as you fell, and you’d end up with your bellybutton roughly where your feet had been previously planted.
Come to think of it, I’m not sure how one would get up if one fell over on such a surface…
I think it’s a little more complicated than you make it sound. When you have to breathe in, would that create a suction that would halt your progress, or does air flow into your mouth from all directions and out in a directed stream? You could turn your head between inhaling and exhaling, but on a frictionless surface, turning your head one way tends to turn your body in the other. I’d try using my hands like the thrust reversers on a jet engine; close them in front of my mouth when I inhale, so the air is drawn in from the sides, then open my hands and blow straight out.
The SF author is wrong; the force needed to throw anything away will create a reactive force that push the thrower in the opposite direction. On a totally frictionless surface, that force could be as small as you please.
Could you even stand on such a surface? I’d think that with absolutely no friction, even something as simple as breathing would throw your balance off just enough to make you fall flat on your rear.
I’m certain that the velocity vectors of the air molecules entering and leaving your mouth are very different - When you inhale, air flows from every direction into your mouth, and when you exhale, it moves in a more-or-less straight line away. My supposition is that this is enough to move you.
It may be difficult, but in theory you can stand on it. You can correct for imbalance by moving your feet across the surface. Kind of like balancing a broomstick on your hand - you can’t move the center of mass of the broomstick, but you can change where it’s supported.
Anything you do that doesn’t interact with the outside world will move you, so if you’re trapped on a frictionless surface in a vacuum, wearing a survival suit without anything you can detach, you’re screwed.
But you can fan the air with your hands and get moving. Or take off your shirt and throw it away. You’ll keep moving until air resistance stops you. Or breathe – blow out forcefully. Sneeze. Burp. Fart (without pants to dissipate, as in our dicussion about visibility). Any of these actions have you pushing something away from you, so you recoil in the other direction.
My freshman physics books had some delightful problems associated with frictionless surfaces. You can derive some fascinating problems from this. For example – you start with a ladder of length L leaning against a frictionless wall and floor (that make a right angle). The initial height of the top of the ladder is h above the ground.The reason it’s not falling now is that you’re standing on a frictional surface nearby and holding it up. You let go. the ladder starts to slide down the wall. at some point it loses contact with the wall and moves sideways, away from the wall. At what height does it do this?
Or: You have a sphere of frictionless material of radius R. You place something at the top and release it. It starts sliding down the outside of the sphere, but eventually it picks up enough momentum to separate from the sphere and fly off. How far from the top does it do this? How does this change if you push it?
That’s only assuming if the underside of the surface is also frictionless.
Even if you can stand on the surface, it would be so difficult that any motion to throw something to propel yourself would knock you off your feet. So now you’ve got to figure a way to get off while lying down. I think swimming like a frog would be best.
Now let’s say that I was actually worried about this, accidentally walking onto a frictionless surface. What would be in my best interest to carry around with me, 2 or 3 bricks so I can throw a lot of mass away from me or like 100 golf balls so I can chuck them harder? My understanding is that both will move you, but which will move you further faster?