Can you stand on a frictionless plane?

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  • #51
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You wouldn't so much keep adjusting them as you would keep them rigid. In theory, if your leg muscles were strong enough, you could plant your feet a metre apart and still stay upright (skaters and cheerleaders can do this). You would not have to "keep adjusting" anything - likewise, a perfectly solid statue of a man would stand like this just fine with no sliding.

But, the problem is, that is not the way we stand. I'd be happy to direct you to a number of postural studies if you're interested.
 
  • #52
russ_watters
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russ_watters - Correct, the vibrations are fairly random;
but the overall effect would not be zero.
Yeah, it would - if it isn't zero, it isn't random. And it is random.
I would guess that the points where there are large masses on your body that are touching the frictionless surface less (ie. your neck and head) would produce the greatest directional difference. And therefore you would probably start moving around somewhat randomly (very small motion, almost imperceptible) and then after time you would pick up speed and after awhile the force pushing back the other way would be negligible.
If the plane is truly frictionless, there can be no force.
You have to realize that the overall net force by the vibrations of the atoms could not be zero, and that there would be one direction which overall would extend an overall greater force on your body and that's the direction you would end up going.
It must be zero.

What you are describing violates conservation laws.
Nice pictures on your site by the way. I hope to get a nice telescope someday. Any suggestions?
Thanks! What you should get depends a on your experience level, budget, and what you want to do.
 
  • #53
russ_watters
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TVP45 - I should have clarified my point a bit more. The adjustments I'm referring to are simply flexing your muscles, bending your waiste, bending your arms and knees, pulling your arms down more in front of you or pushing them up. You could still make all those movements. Indeed there would be less resistence because there is no friction from the surface contact. Imagine a man (this time on all fours - hands and feet) on very slippery ice. He can use his muscles to pull his limbs closer together in a tighter formation, or relax them a bit and let gravity move his limbs, spreading them out, away from each other.
That's true, but none of those movements can cause a person's COG to move. An astronaut in space cannot propel himself just by jerking his arms. That's a violation of conservation of momentum.
So you are saying that every atom in our body would vibrate with the exact same force, no matter what pressure is applied to it and not matter what kind of atom it is? That seems a bit ridiculous doesn't it?
I'm not sure what "pressure" you are talking about, but what we're saying is that every atom vibrates in a random direction and at a kinetic energy based on it's temperature. Non-random group motion such as flexing muscles is different, but such motions do not create external forces, only internal ones.
Heck, since they didn't say that there was no air, then even breathing in one direction continuously could start you moving.
Again, via conservation law, breathing in and out in the same direction would cause you to oscillate back and forth, but could not cause net motion. Danger pointed out that you could turn your head one way and inhale, turn it another and exhale - that would cause a net propulsive force. So the bottom line is you have to be clear on the difference between internal and external forces and motions.
 
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  • #54
russ_watters
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But, the problem is, that is not the way we stand.
Agreed. People aren't tables and we aren't rigid. Any instability - any tiny twitch of your muscles would cause you to lean one direction (a gravity-induced rotation) and with no friction, your feet will go right out from under you when you attempt to compensate by applying a force on the ground in the opposite direction.
 
  • #55
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Remember why air is going into the mouth - a low pressure area is created in the lungs.

That low pressure is transferred to the volume in front of your face by way of your throat. So the air in your throat is under lower pressure, not higher pressure. And your body is pushed into that low pressure area by the higher pressure behind your head.


When a fan sucks air through itself, does the air "hit" the back of the fan blades propelling the fan forward? No. The fan is pushed backward into the partial vacuum.

I think, still, force is by pressure and area.

and anyway, I think, also, that your head would rotate around your C of G, and your feet would slide in the opposite direction. (you'd have to lean perfectly on each breath)
 
  • #56
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DaveC426913 - That's more or less what I meant. You could keep your legs and arms fairly rigid; but you would tire and need to readjust so as not to collapse or fall over.

DaveC426913 - Maybe I'm viewing this wrong; but this is a frictionless surface right, and we're ignoring any other force; but gravity, right? So any atoms that are against the ground, that vibrated at any downward angel, then for that moment they would push against absolutely nothing (unless they were exactly perpendicular to the plane), whereas all of the other atoms would actually be pushing on something (whether it's air molecules or other atoms which makeup the body).

DaveC426913 - I can see what you mean about the breathing thing; but that's only if you breath in at the exact same angle as you breath out, because again, any force at a downward angle would be less than if it were parallel to the plane... Ok, looks like you discussed that a bit. Even if your path was not straight you would still be accelerating and wouldn't even notice a direction change. Wow, this breathing thing is fun. So, turn your head 90 degrees (not 180) and then bend your head down so that if someone was in front of your body they would see the top of your head (the beginnings of a bald spot in my case). Then, as you breath in through your nose, you would be pulling in air from behind you, thus making you move in that direction (backwards). Then expell your air directly in front of you with your mouth, thus making you move backwards again.

DaveC426913 - I'm not sure on the max speed, you bring up a good point; but I would still have to disagree. If there was no overall motion of the air (no wind), then you're saying that you wouldn't go anywhere. I think that as you expell air the act of expelling is what is pushing you in the opposite direction, not your expelled air pushing against air molecules.

DaveC426913 - Ooo, Dave, you've got a great idea. Use a fan. :D Hold that above your head, or two of them off to your sides and start racing along. :D Talk about a cost efficient, environment friendly fuel. :D

russ_watters - So, let's make the atoms into coins. I'm not sure how many billions of coins that is; but what you're saying is that flip all those coins and they will exactly balance out?

russ_watters - True, I wasn't meaning to say that using your muscles to move your arms and legs would give you a net movement. Sorry if that seemed implied.

russ_watters - Oh, sorry, the pressure I was talking about was just your body (made up of atoms) resting on a frictionless surface.

russ_watters - My budget will be about $1,000 (I've been saving for years). I have only viewed objects through other's telescopes, and therefore I haven't taken any pictures using a telescope.

russ_watters - Yep, I'll try to be more clear on my forces. Though turning your head would start your body spinning wouldn't it?
 
  • #57
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The "fly in the ointment" in this discussion is that ice is a "very slippery" surface and people are using that as a reference. It is not very slippery except under special circumstances, i.e., kinetic and high pressure. Otherwise, you can expect a static coefficient of about 0.1.
 
  • #58
DaveC426913
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But, the problem is, that is not the way we stand. I'd be happy to direct you to a number of postural studies if you're interested.
1]The question was not "is it possible to stand comfortably and naturally on a frictionless surface?" The question was "is it possible?"

2] The point I'm making is that we can even go to extremes and it still works in priniciple. That it's not practical I would have thought went without saying. But apparently not.

Danger pointed out that you could turn your head one way and inhale, turn it another and exhale -
Hey! That was me.

as you breath in through your nose, you would be pulling in air from behind you, thus making you move in that direction (backwards).
Not quite. The air would come in the sides; the net pressure would be inward from both sides; thus no forward movement. However, the technique would still work, you'd just only move on the exhalation, not on the inhalation.


I'm not sure on the max speed, you bring up a good point; but I would still have to disagree. If there was no overall motion of the air (no wind), then you're saying that you wouldn't go anywhere.
No, I did not say that at all.

I am simply saying that your velocity will top out at the velocity of the expulsion of your breath. i.e. If you can expel your breath at 4 mph, then eventually you could reach 4mph, but no faster. I'd like to hear from some propulsion experts on this though.


russ_watters - So, let's make the atoms into coins. I'm not sure how many billions of coins that is; but what you're saying is that flip all those coins and they will exactly balance out?
It doesn't matter if they balance out.

Let's turn the atoms into human limbs that can create a force of 50 pounds. You stick an arm out in one direction with a force of 50 pounds. What happens? Your body moves a bit in the opposite direction and stops. You are nopw standing still, with one arm outstretched. While your torso and legs may be displaced by an inch or two, your centre of mass has not budged, and you are still motionless. Extend your arms and legs randomly, in any direction you want - it will not affect your CoM and it will not impart any motion on you.

This is why astronauts, if separated from their capsule with zero relative v. are doomed - unless they have some sort of propulsion.


Though turning your head would start your body spinning wouldn't it?
It would turn your body until you stopped turning your head - and then your body would stop rotating, leaving you with zero spin.

Just like an astronaut cannot move himself toward his capsule, it also means he can't start (or stop) himself rotating.
 
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  • #59
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DaveC426913 - Good point. I'm not sure about the max speed caused by expelling your breath. I guess I'm viewing it as adding velocity each time you breath out; but it might be as you describe it, capping out at the max expulsion rate of your breath (which could be pretty quick).

DaveC426913 - I see what you mean by the force and movement due to atoms; but instead of just holding your arms out you would be moving your arms in and out repeatedly, which in a vacuum (astronaut in space) nothing would happen; but now imagine tons of big rubber balls flying everywhere. As you move your arms in and out you hit the rubber balls at times, and that would push you back. See what I mean. Now the question is just if the outer atoms which makeup your body, would be exactly equaled out by the force they exert on the air molecules. Now if you had a perfectly symmetrical object, then they should equal out; but our bodies are not perfectly symmetrical.
 
  • #60
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1]The question was not "is it possible to stand comfortably and naturally on a frictionless surface?" The question was "is it possible?"

And, that is the question I answered. It is not possible, although a few trained athletes, e.g., skiers, who use knee and hip strategy can probably do it for perhaps 15 or 20 seconds. Postural Sway studies have nothing to do with comfort. Again, I will gladly point you toward some of those papers if you're interested.
 
  • #61
disregardthat
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1st case (train is accelerating)
the person cannot stand still on the floor, because there is no force acting on the person to make it accelerate with the train as well.
,
was said some pages back.

Even if the surface under you is accelerating it wouldn't mean anything since it is frictionless, right? The same properties stays true.
 
  • #62
russ_watters
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russ_watters - So, let's make the atoms into coins. I'm not sure how many billions of coins that is; but what you're saying is that flip all those coins and they will exactly balance out?
Not exactly, but the more you flip the closer they are likely to get to exactly balancing out, percentagewise.
 
  • #63
DaveC426913
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And, that is the question I answered. It is not possible, although a few trained athletes, e.g., skiers, who use knee and hip strategy can probably do it for perhaps 15 or 20 seconds.
How can you, in a single sentence, claim something is not possible and then immediately explain the circumstances in which it could, in fact, be done???

Possible means possible.
 
  • #64
DaveC426913
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As you move your arms in and out you hit the rubber balls at times, and that would push you back. See what I mean.
Yes, by moving your limbs, you can act upon the air. You are effectively swimming.

But you will only make headway if you coordinate your movements. Otherwise you will go nowhere.

Actually, that's not quite true. You will perform what is called a random walk.



Now the question is just if the outer atoms which makeup your body, would be exactly equaled out by the force they exert on the air molecules. Now if you had a perfectly symmetrical object, then they should equal out; but our bodies are not perfectly symmetrical.
Non-symmetricality does NOT lead to non-symmetrical forces.

Think of an abandoned rowboat in a lake that has no current. The random forces of the water do not cause the rowboat to start moving. If this were true, you would have discovered a free energy source.


BTW, this is not just theory. A few years back, when nanotechnology came into the fore, someone invented a nano-scale motor-ratchet device that took advantage of the very forces we are speaking of, namely, Brownian motion. The idea was that the random motions of molecules bouncing off the device could be used to turn the axle, but a ratchet converted the random motion into a one-direction turn rather than simply a back-and-forth motion.

Indeed, if the device had worked, they would have invented a perpetual motor. Alas, physicists using Newton's laws of motion were able to demonstrate that the device could not work.
 
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  • #65
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Jarle - True, if the frictionless surface below you was accelerating it would make no difference. I haven't read the post you reference; but it sounds like they might be trying to add mass to make it easier to stand. That would have null effect as far as I can tell.

russ_watters - True; but those forces on the outside of the body touching the frictionless surface and exerting a force in any kind of downward angle (except exactly down) would have null effect. Whereas the forces on the outside that are not touching the frictionless surface are in essence the only ones we need to pay attention to. Now with a completely symmetrical body you would have the same amount of force exerted everywhere; but thankfully my body is not a perfectly symmetrical object and so the balance would already be tilted.
 
  • #66
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DaveC426913 - Well, then maybe I'm wrong; but it seems (and this is an exageration to display the point) that if you have 100 arms on one side hitting flying balls and only 50 arms on the other side, then you would start moving in the direction where the 50 arms are pushing because their overall force is canceled out by 50 arms from the other direction; but then there are still 50 more arms. I guess the object would not only have to be symmetric; but also of uniform elemental makeup.

P.S. I enjoy discussing this with you all. It makes me think.
 
  • #67
DaveC426913
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DaveC426913 - Well, then maybe I'm wrong; but it seems (and this is an exageration to display the point) that if you have 100 arms on one side hitting flying balls and only 50 arms on the other side, then you would start moving in the direction where the 50 arms are pushing because their overall force is canceled out by 50 arms from the other direction; but then there are still 50 more arms.
Yes, that's the coordinated part - 50 arms all pushing against the air in one direction.

Replace the 50 arms with one fan-shaped membrane, and the air with water - and you've got a swimming fish.
 

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