Running on water

[FungusForge]

Running on water is impossible on Earth, at least without ridiculous footwear. We all know this. But could someone run on water on the moon or maybe even Mars?

This is of course assuming it were a climate controlled indoor environment.

[sevenperforce]

According to this paper linked in one of Randall's What-Ifs, yes.

[lajoswinkler]

No, never. Even with low gravity, you couldn't do it because you have mass that you need to push away from a surface, and you can't push against water. You could walk on it, very slowly (and I mean VERY) in very low gravity if you'd rely on surface tension.

[sevenperforce]

2 minutes ago, lajoswinkler said:

No, never. Even with low gravity, you couldn't do it because you have mass that you need to push away from a surface, and you can't push against water. You could walk on it, very slowly (and I mean VERY) in very low gravity if you'd rely on surface tension.

You can't push against water?

You can most certainly push against water.

Pushing against a fluid is how every rocket works.

It's even easier with an incompressible fluid like water.

[lajoswinkler]

Just now, sevenperforce said:

You can't push against water?

You can most certainly push against water.

Pushing against a fluid is how every rocket works.

It's even easier with an incompressible fluid like water.

Rockets don't work like that. They throw a lot of mass at very high speeds which causes a reaction force that pushes the rocket in the opposite direction. It does not rely on electromagnetic interactions between particles.

[sevenperforce]

1 minute ago, lajoswinkler said:

Rockets don't work like that. They throw a lot of mass at very high speeds which causes a reaction force that pushes the rocket in the opposite direction. It does not rely on electromagnetic interactions between particles.

And how, pray tell, do you imagine they "throw a lot of mass"? And what do you think "pushes the rocket in the opposite direction"?

The expanding fluid coming out of the nozzle choke pushes against the exhaust bell, and the exhaust bell pushes back against the fluid.

Water cannot support your weight, but it has inertial mass just like anything else, so you can run on water if you can push down on it hard enough and fast enough and keep going.

[FungusForge]

27 minutes ago, lajoswinkler said:

...and you can't push against water.

Uh, I'm not expert in any scientific field (hence why I come here and ask these questions), so correct me if I'm wrong, but when I skip a stone on a lake, is that stone not pushing against the water?

[lajoswinkler]

 

1 hour ago, sevenperforce said:

And how, pray tell, do you imagine they "throw a lot of mass"? And what do you think "pushes the rocket in the opposite direction"?

The expanding fluid coming out of the nozzle choke pushes against the exhaust bell, and the exhaust bell pushes back against the fluid.

Water cannot support your weight, but it has inertial mass just like anything else, so you can run on water if you can push down on it hard enough and fast enough and keep going.

You're mixing two processes here. The engine bell serves a purpose to enhance, boost the efficiency.

This is how rockets work.

If your explanation was valid, no nozzle would mean no thrust, and there are plenty of cases where nozzle is absent and pure reaction force develops.

 

 

53 minutes ago, FungusForge said:

Uh, I'm not expert in any scientific field (hence why I come here and ask these questions), so correct me if I'm wrong, but when I skip a stone on a lake, is that stone not pushing against the water?

No, it already has a momentum (and a big one) and it's just gliding against water.

Edited March 11, 2016 by lajoswinkler

[sevenperforce]

Just now, lajoswinkler said:

You're mixing two processes here. The engine bell serves a purpose to enhance, boost the efficiency.

This is how rockets work.

 

 

No, it already has a momentum (and a big one) and it's just gliding against water.

You're...wronger and wronger.

What is an equal-and-opposite force if not a push?

And what do you imagine "gliding against water" means?

Do you have any background in basic physics, free-body diagrams, anything like that? Because it's really easy to see. Take the example of the rock skipping across the lake. The rock does not remain in contact with the water; it bounces. What is pushing the rock back up, if not the surface of the water?

[razark]

If you can't push against water, how do oars and propellers work?

[lajoswinkler]

I suppose you place the rock on the water and it hops around like a frog?

[kerbiloid]

3 hours ago, FungusForge said:

Running on water is impossible on Earth

 

[Shpaget]

While I don't consider the monk example a good one, I have seen plenty of videos of cars, motorcycles, quads and even snowmobiles driving on water, for quite a distance.

[AngelLestat]

Newton´s third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

I am sure that you will be able to run on water on the moon, but with mars gravity.. mmm not sure if is possible.

[KerikBalm]

you most certainly can push against water... its called swimming.

And a rocket works because of the pressure of the gas along the inside of the engine bell and combustion chamber. Its directly linked to the action and reaction. there is a force acting on the gas pushing it backwards, and equal and opposite force is acting on the combustion chanber and nozzle pushing the rocket in the other direct.

Likewise, a mass driver (lets say its a coilgun) will experience a force on its magnetic coils, equal to the force the magnetic coils exert on the projectile. Its a matter of reaction mass for sure, but there are forces involved: magnetic in the case of a coil gun, and direct gas pressure in the case of a rocket.

That balloon goes forward because there is pressurized gas inside (the pressure being generated by the contraction of the elastic membrane), but due to an opening in one side, there is a net force in one direction from that pressure. That is *directly linked* to the expulsion of reaction mass. Essentially both explanations are correct. Your explanation is not wrong, but by saying his is wrong, you yourself become wrong.

[sevenperforce]

1 hour ago, AngelLestat said:

Newton´s third law: When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

I am sure that you will be able to run on water on the moon, but with mars gravity.. mmm not sure if is possible.

The internationally-funded, multiple-PhD paper I linked above said that you can run on water on the moon, barely.

3 hours ago, lajoswinkler said:

I suppose you place the rock on the water and it hops around like a frog?

Not place, throw.

But yes. Yes, it does. Have you ever skipped a rock across a lake? Hopping in a very frog-like fashion is exactly what happens. Are you just trying to troll us?

Edited March 11, 2016 by sevenperforce

[FungusForge]

I do beleive I have gotten my answer (which happens to be a yes).

@sevenperforce

BTW, thanks for linking to that paper, its a good read.

@lajoswinkler

It seems you misunderstand inertia. An object in motion will stay in motion unless acted upon. Skipping requires a change in direction the stone travels, if the water couldn't be pushed on (and push back as old Newton would suggest), then the stone's momentum would carry it in a straight trajectory to the ground below the water. It would not make a change in trajectory or velocity.

I'm sorry but I think you are incorrect in the belief that being incompressable means that water cannot be physically interacted with. Everything from rowboats to nuclear aircraft carriers physically manipulate (aka push) water to move across bodies of water.

 

[AngelLestat]

4 hours ago, sevenperforce said:

The internationally-funded, multiple-PhD paper I linked above said that you can run on water on the moon, barely.

haha, they had time to made this research?
Ok I did a quick read, but I don't find where it said that this is possible in mars.   THe graphic show 22% earth gravity and 2.4 m/s vertical velocity, that is no enough for mars.
 

[Snark]

Snowmobiles can run across water just fine, and they're certainly not relying on buoyancy to do so.  They just have a whole lot of engine power.

The thing that stops people from running on water isn't any fundamental law of physics, it's an engineering constraint of what human muscles are capable of.  Turn gravity down, the power requirements to skim it go down.  It's just a question of how far down it would need to go.  The paper referenced indicates that the Moon's gravity is right around the borderline of what humans could handle.

(And I love the photo of their "test laboratory" in the paper.  Am I the only one who imagines the guy's phone ringing and he picks it up and says breathlessly, "I can't talk now, Mom, I'm doing SCIENCE!")

[Windspren]

I can walk across water. I can even sit, or stand on it. Of course, this only works when its really cold outside, but hey, ice is still water!

[YNM]

Running on water... Our feet is quite hydrodynamic compared to fins and shoes, so bare-footed it's a hard thing.

Snowmobiles on the other hand, they have scoop-like (or rigid flap-like) features on their "tyres" (um, what is it called), not to mention it have larger surface area wrt feet, so keeping the majority of it aboce water isn't as hard. You just need to keep the revs high. Unlickily, I don't think our feet can reach the required rpm for it to near-continouosly skim the water surface...

Stone skipping is a different thing - they're light, and when the water splash around there's more contact zone (in fact, almost surrounding) to the rock for the water surface to deflect it.

Same thing, your feet just need to skim the water fast.

 

 

[AngelLestat]

6 hours ago, Laythe Dweller said:

I can walk across water. I can even sit, or stand on it. Of course, this only works when its really cold outside, but hey, ice is still water!

No, water is the liquid form of H2O and ice is the solid one, there is no way to go around that

[lajoswinkler]

6 minutes ago, AngelLestat said:

No, water is the liquid form of H2O and ice is the solid one, there is no way to go around that

It is water, if one is talking about chemical composition.

[AngelLestat]

1 minute ago, lajoswinkler said:

It is water, if one is talking about chemical composition.

http://chemistry.about.com/od/chemistryglossary/g/water-definition.htm

Water Definition: Water is a chemical compound consisting of two hydrogen atoms and one oxygen. The name water typically refers to the liquid state of the compound. The solid phase is known as ice and gas phase is called steam.

 

[lajoswinkler]

2 minutes ago, AngelLestat said:

http://chemistry.about.com/od/chemistryglossary/g/water-definition.htm

Water Definition: Water is a chemical compound consisting of two hydrogen atoms and one oxygen. The name water typically refers to the liquid state of the compound. The solid phase is known as ice and gas phase is called steam.

 

And how is that different from what I've said?