[alancalverd]

And now I'm wondering about the optimum trajectory of someone who jumped inwards from the surface of a rotating space station. Time to get a life, perhaps?

[gem]

Hi all,

Halc, yes I take your point regarding gear equivalence for skating technique, its not my strong suit, think Bambi on ice and you have a pretty good visualization.

Take a look at the link below it covers some of the points as to the physics.

It does state, the mechanical advantage of the  pedal system of gears does allow a much greater Tyre speed vs leg speed.
 
https://physics.stackexchange.com/questions/505636/what-determines-the-top-speed-in-ice-skating

I believe someone raised the point of the similarity with sailing dynamics as to forward motion gained from a side on wind.
 

[JesWade21]

I've been able to run in the simulation of Martian gravity (in an electronic harness). I've gained an immense amount of empathy for lunar astronauts. However, even in Mars gravity (that's what, 0.4g? ) You can't even perform a move, you must hop. There's just not enough friction to guarantee yourself a horizontal push.

Because your mass doesn't change in relation to gravity Any rise in gravity will raise the friction of your body, but not increase your inertia. In contrast any reduction in gravity will reduce your friction but not diminish your inertia.

Also, there is the issue that gravity increases, resulting in greater atmospheric pressure. This means that more air is pushing through, which slows your speed. The math required to determine how this is compared to the benefits of friction that is greater with the ground is beyond my comprehension.

[Arinece]

Not everyone can run or move fast!

because you have to practice it,

The human body does fight with the air pressure that exists in the way to move a step during running.

and a human can make faster decisions than a computer system. Even it can deal with the great instruct system with other body parts.

[Halc (OP)]

Special skates going off the sides of the feet..

Special skates for people need to keep the blade centered on the leg, so my comment above is nonsense. It might be different for robots.
If a person wants to maximize horizontal thrust to lift ratio, one uses a taller skate such as is used by short-track racing, and not a skate with an off-center or tilted blade.

That reduces friction significantly (but not to zero, because then they would have no acceleration)

That's like suggesting that a rollerskater could not accelerate if the wheel-bearings were frictionless. Of course speed skaters strive for (however unattainable) zero friction in the direction of motion. That's the point.

On the Moon, the gravitational force on a human body is about 6 times less than on Earth, so this would require a skate contact area that is 6 times less - maybe 3 times shorter, and half of the width? (Assuming normal atmospheric pressure, so the ice doesn't sublimate.)[/quote]Again, disagree. To apply the corresponding pressure needed to melt the ice, it would be proportional to the force applied to the skate, not proportional to the weight of the skater. You're assuming the guy is just standing there, but he's applying much more force than that. A skater on the moon would want a much lower angle of attack to minimize upward lift, so the taller short-track skates would prevail. He might be able to apply (guessing) half the pressure that he does on Earth, so maybe half the contact area, but it would be an awkward gait. The optimal speed might be achieved by skating in a circle instead of a straight track.
Also, maybe a longer skate would help prevent slippage to the side.

I believe someone raised the point of the similarity with sailing dynamics as to forward motion gained from a side on wind.

Sailing utilizes energy from a difference in velocity of two different mediums. That makes it a poor analogy to skating where there's only the one medium and the work is done against one's own inertia rather than the second medium.

However, even in Mars gravity (that's what, 0.4g? ) You can't even perform a move, you must hop. There's just not enough friction to guarantee yourself a horizontal push.

Hopping is more efficient than walking, and running probably isn't viable at all, lacking a way to counter the torque of alternating legs being used in long strides. A horizontal push is not hard to do, but one sufficient to move quickly is difficult, yes.

The question is, could my max speed be higher at 0.9g or 1.1g?  Some gravity where running (for which we're evolved) still works. Where's the sweet spot?  I agree that 0.4g is probably too low and we'd be slower on Mars.

Any rise in gravity will raise the friction of your body, but not increase your inertia.

Agree, but how much is that friction useful once you're already up to speed?  That friction seems most useful only for the initial acceleration/cornering.

Also, there is the issue that gravity increases, resulting in greater atmospheric pressure.

4th time: Please read the OP and subsequent posts. No change in atmosphere.

This means that more air is pushing through, which slows your speed.

More time airborne between strides increases the duration that the air friction works against you. But the pressure is assumed invariant at any gravity.

Not everyone can run or move fast!

Besides the point. I'm talking about an average healthy person, and perhaps one who has had time to practice in the altered gravity, but not so much practice that his physiology has changed.

and a human can make faster decisions than a computer system.

Not so. A computer can react many times faster than can a human, and it has been that way for a while now. The playing card trick shows this: Have somebody hold a playing card vertical, with you holding your fingers a cm apart with the bottom edge of the card between them. When the person lets go of the card, trick is to clamp down, catching the card before it passes between your fingers. Apparently it's beyond human capability, but a robot can do it before it moves even a 10th of its height.

[evan_au]

a rollerskater could not accelerate if the wheel-bearings were frictionless

The critical factor with roller skates is the friction between the rubber wheels and the ground, which is intentionally quite high.
- By pushing off at a right-angle to the other roller skate, the friction between rubber and ground holds one foot in place, the bearings of the stationary foot do not rotate, while achieving a good speed with the other skate.
- Repeat with each foot alternately;
- (like ice-skating) as speed increases, push with a narrower angle between the skates.

[Eternal Student]

Hi.

Found this:
https://journals.biologists.com/jeb/article/221/3/jeb162024/20344/Reducing-gravity-takes-the-bounce-out-of-running

A scientific study, with actual data collected, about the running stance and gait adopted in simulated low-G environments.

Their conclusions:  Mainly that the gait is adjusted so as to keep the centre of mass extremely flat and level, i.e. almost all bounce is removed.  Their models suggest this is energy efficient (they use "an impulsive model of running" developed by Rashevsky and Bekker - although these people contributed at different times and not collaboratively).
    There is no comment or investigation about the maximum speeds attainable, sorry.  However, if this low bounce method is more energy efficient you would have thought that a runner can sustain a higher maximum speed.

Noteable limitations
   They didn't seem to have a wind fan or anything to re-create the effect of air resistance.  However, you would have thought this would only further reduce speed while in the air and unable to provide propulsion with your feet, so that it would only increase the desirability of maintaining a low bounce running style.
- - - - - - - - - -
We've already mentioned that accelerating from a standing start is a completely different thing to sustaining a high top speed.   Just for amusement, here is Usain Bolt trying to sprint in low G:

Best Wishes.

[alancalverd]

Pressure and oxygen level is being held normal at any g, per the OP. Why do I have to say this a third time?

Because it can't be done unless you put your athlete in a diving bell. So maybe it can be done. I had envisioned the guy running in free air on an alien planet, in which case you need to consider physics and physiology. Apologies for being so narrow-minded.

[Bored chemist]

Because it can't be done unless you put your athlete in a diving bell.

Why not?

[Petrochemicals]

Best Wishes.

Given infinite time what speed would he achieve. If 99.999etc of his energy was directed forward rather than up I surmise this must be fastest, sort of like a n ion drive, slow but steady.

https://en.m.wikipedia.org/wiki/Ion_thruster

[alancalverd]

Quote from: alancalverd on Today at 20:31:13
Because it can't be done unless you put your athlete in a diving bell.

Why not?

See # 13 and 16 above.

[Petrochemicals]

from: alancalverd on Today at 20:31:13
Because it can't be done unless you put your athlete in a diving bell.
Why not?

See # 13 and 16 above.

Talking to yourself again Alan..?

[Bored chemist]

Quote from: alancalverd on Today at 20:31:13
Because it can't be done unless you put your athlete in a diving bell.

Why not?

See # 13 and 16 above.

Given that it's me asking, do you want to think it through a bit?

[Bored chemist]

The Royal Air Force, Royal Navy, NASA and ROSCOSMOS have all thought it through, just to mention a few organisations interested in keeping folk alive in various atmospheres. The trick is to maintain around 200mb partial pressure of oxygen to keep the squishy bits working, with an ambient pressure between 0.2 and 2 bar so that the diaphragm muscle can do its stuff.

And somehow, the only way to do that is a diving bell?
Why not just find some where with the gravity you want and also the atmosphere you want?
(Obviously, it's a thought experiment so I can do that).

[alancalverd]

The Royal Air Force, Royal Navy, NASA and ROSCOSMOS have all thought it through, just to mention a few organisations interested in keeping folk alive in various atmospheres. The trick is to maintain around 200mb partial pressure of oxygen to keep the squishy bits working, with an ambient pressure > 0.2 bar so that the diaphragm muscle can do its stuff, and a diluent gas that doesn't significantly invade the squishy bits above 2 bar.

Now if you take a normal atmosphere of 4:1 nitrogen:oxygen and increase g, the surface pressure on your planet will increase unless you do something to remove the upper layer of gas, in which case you aren't dealing with a normal equilibrium atmosphere.

What the aforementioned clever folks do is to introduce pressurised cockpits, hard-shell submarines and diving bells according to the problem at hand.

[Bored chemist]

Do you really think that?
Do you realise they were mainly interested in this planet?
They won't have considered varying g much.

[Bored chemist]

Ok Imagine we leave g constant- because it is easy.
Why not just vary the pressure until you get the value you want?

[alancalverd]

Your thought experiment could, for simplicity, include  taking the required atmosphere in a can to the moon or Jupiter, which would be a lot simpler than scouring the universe for a planet whose atmosphere defies the laws of physics.

[alancalverd]

Why not just vary the pressure until you get the value you want?

That is indeed the function of pressurised cockpits and pressure hulls. And the OP  specifically requested 4:1 at 1 bar.

[Bored chemist]

Do you think that g determines the pressure?
That seems to be what you are saying.

I'm off to bed.
I invite you to consider what would happen if a passing alien spacecraft stole half our atmosphere.
There wouldn't be much change in g, and what was left would still be 21% oxygen etc.
But it sure as hell wouldn't exert 14.7 PSI at sea level.

So it's clear that you can adjust the air pressure independently of the local gravity.
No diving bell required.