Downwind faster than the wind

[Rohit507]

i'm suprized this topic hasn't come up yet here.

Now the question is : Can you build a machine that moves downwind faster than the wind, without using anything but the wind as a source of energy?

i'm convinced that you can because of a working experiment ( http://www.youtube.com/watch?v=aJpdWHFqHm0 ) and this ideal model ( http://www.youtube.com/watch?v=k-trDF8Yldc )

there's also this ( http://www.youtube.com/watch?v=xHsXcHoJu-A ) working against various other claims of how this coudn't work, and providing an overview of the problem.

now, that should be enough to see that you can build such a machine, but i still have no idea why?

intuitively it makes no sense, and with my limited physics background it makes no sense.

[Diadem]

Assuming constant wind, and you going exactly downwind, then if you move with exactly the speed of the wind, the air will be stationary compared to you. Thus you can't get any energy or momentum from it. Thus you won't be able to accelerate, and friction (with the ground / water, not the wind) will in fact decelerate you.

You could move sideways a bit, instead of going exactly downwind (sailing ships in fact like to do this, they go faster with the wind comming at an angle). But that still won't fix this problem. You can look at both components of your velocity (parallel and perpendicular to the wind) seperately, and see that you can never gain momentum in the parallel direction once your parallel velocity is equal to the windspeed.

So no, you can't.

[Diadem]

I thought of an even simpler way of seeing this.

Make a coordinate transform to the rest-frame of the wind. You'll see that in the case of no friction with the surface you're on, claiming you can move faster than the wind on windpower alone is equal to claiming that you can move on windpower if there isn't any wind at all!

Clearly an absurd claim.

[Hawknc]

...I like this thread. This is making me think pretty hard about whether it will work or not. Bear with me because this is probably going to be a bit of a stream of thoughts rather than anything coherent.

(Disclosure: I studied aerospce engineering at uni but I'm not presently working in aerodynamics or propulsion, so anything and everything I say could be completely off-base. However, I have my doubts that this requires much aerodynamics at all.)

The theory behind this actually working seems to lie in the fact that you can't assume zero friction with the ground. In this case the wheels are not there to reduce friction but to drive the vehicle, which requires traction. The second video the OP linked describes the theory quite well - the vehicle moves more than twice as fast as the ruler, much to the teddy bear's surprise.

Going back to basics and doing a force balance, though, it's not clear where the propulsion force is coming from. The first video shows a vehicle with a large propeller, and the linked PDF states that the wheels are turning the propeller and that the propeller need only produce enough "lift" (I'm fairly sure he means thrust in this case) to overcome the forces required to run it. So, at speed, if the wheels are driving the propeller, what's driving the wheels? The momentum by itself can't drive the wheels indefinitely because it would have to slow down to avoid violating conservation of energy. This is where the link between the first and second videos falls down: one can easily explain a higher speed than the ruler with gearing ratios, as the setup is basically just a rack-and-pinion gear system. It still maintains conservation of energy. The same can't be said for a propeller in the wind, which needs to be driven by something in order to produce thrust. Assuming the wheels don't have their own external energy input, where is the energy coming from to drive the propeller?

[gmalivuk]

You'll see that in the case of no friction with the surface you're on, claiming you can move faster than the wind on windpower alone is equal to claiming that you can move on windpower if there isn't any wind at all!

Clearly an absurd claim.

Yes, clearly absurd. Of course, that's probably why no one's making that claim. The wheels are necessary for this to work, so you can't assume no friction with the surface when trying to simplify what's going on. And the moving by windpower without wind, well, that's what the device on the treadmill is supposed to have done. In other words, please actually watch the videos before deciding why it can't be done. (Your claim that it's absurd would apply equally to tacking, for instance, or really to going any direction in a boat other than that of the wind itself. But then you go on to say yourself that this is possible. And it's only possible precisely because there *is* resistance provided by the water itself.)

For my part, I see no a priori reason to rule this out on the grounds of conservation of energy. Because energy is being provided, by the wind moving past the ground. It's not perpetual motion or free energy, and doesn't claim to be, because it only moves as long as there's wind or as long as they're running the treadmill. In this way, the ruler example is illustrative. Because it shows how something moving at a particular speed can provide a seemingly counterintuitive amount of energy to the car, making it move faster than that.

[BlackSails]

You could store windpower in some sort of storage device, and then drive yourself off the storage advice once you reach windspeed.

[Diadem]

You'll see that in the case of no friction with the surface you're on, claiming you can move faster than the wind on windpower alone is equal to claiming that you can move on windpower if there isn't any wind at all!

Clearly an absurd claim.

Yes, clearly absurd. Of course, that's probably why no one's making that claim. The wheels are necessary for this to work, so you can't assume no friction with the surface when trying to simplify what's going on.

You're saying that friction is actually *increasing* the speed of your vehicle? That'd be something new! Quickly, go claim your nobel price! Seriously, this is impossible. Friction always works against your direction of motion. That's pretty much the definition of friction.

And the moving by windpower without wind, well, that's what the device on the treadmill is supposed to have done. In other words, please actually watch the videos before deciding why it can't be done.

I admit I hadn't actually watched the videos before. I have now. The device on the threadmill looks convincing, but that only proofs it's a pretty elaborate fake. The only force on the vehicle is the force applied by the threadmill on the vehicle, and that's in a backwards direction. The cart can't go forward. There's no force pushing it forward. Draw a force diagram of the situation. The cart is stationary, so there's no force from the air. Only force is from the threadmill. And that's clearly in a backwards direction.

Your claim that it's absurd would apply equally to tacking, for instance, or really to going any direction in a boat other than that of the wind itself. But then you go on to say yourself that this is possible. And it's only possible precisely because there *is* resistance provided by the water itself.

No, tacking and beating have nothing to do with the resistance provided by the water. When air molucules hit a sail, they don't hit it and loose all their energy and remain stationary. They are reflected. So the momentum transfered to the sailing vessel is the original momentum MINUS the new momentum. Call the direction your ship is moving in x. By positioning the sail just right, you can make sure x component of the new momentum is actually bigger than the x component of the old momentum. So a negative momentum is transfered to the boat. But negative in the direction the wind is comming, so postive in the direction the boat is moving! The downside is that the momentum in the y-direction transferred to the boat is HUGE. Here friction comes into play. By making the boat oblong instead of round, you can make it's friction in the y-direction huge, while keeping it's friction in the x-direction small. So it will move forward more than it will move sideways.

I could explain it more clearly with a picture. Should be able to find one with google though.

For my part, I see no a priori reason to rule this out on the grounds of conservation of energy. Because energy is being provided, by the wind moving past the ground. It's not perpetual motion or free energy, and doesn't claim to be, because it only moves as long as there's wind or as long as they're running the treadmill.

It's easier to see where things go wrong by looking at the forces involved. Or looking at the momentum transfer. Energy conservation is violated, but it's harder to see. We could see it for example by pointing out that in the threadmill scenario, a freely rotating perfect treadmill doesn't require any energy to power. Put the cart on it, and the cart, since it's moving forward, will actually accelerate the threadmill. Tada perpetual motion.

In this way, the ruler example is illustrative. Because it shows how something moving at a particular speed can provide a seemingly counterintuitive amount of energy to the car, making it move faster than that.

The ruler example is completely different. The ruler is an external force that isn't bounded by anything. The movement speed of the ruler doesn't say anything. The fuel in a car ain't moving at all, yet still powers the car. Or look at system of ropes and wheels (can't think of the correct English term for it) used for lifting heavy stuff. You can lift something a 1000 kilos with barely any effort. But you'll need to move dozens of meters of rope to lift it 1 meter. You can also invert this, and use it to pull something upwards very very fast, but you'll need a huge force to move a small object. Either way it's a situation completely uncomparable to wind.

[Goemon]

Ten minutes ago, I would've said that's impossible. But after watching the second video and imagining the ruler = the wind and the paper = the ground, now I have to think about it...

[Mr. Beck]

After looking at both videos, I''m beginning to see how this is indeed possible.
First off, can we all see how a treadmill is equivalent to a wind across a flat surface? If not, ask for clarification.
As we begin, all of the device's components are stationary. As soon as it is placed upon the treadmill, the mass/inertia of the vehicle causes it to stay in pretty much the same place, at least for a bit. However, the wheels can still rotate freely underneath the car. Their rotation provides power to the propeller, which pushes the car forward. Already, it is moving "faster than the wind". Even then, as soon as it moves forward the speed difference between the car and the surface increases, providing yet more power to the prop.
It's not the propeller powering the wheels, it's the wheels powering the propeller. In fact, this problem is quite similar to POATM.
In neither case is there perpetual motion. As soon as the wind/treadmill stops, the car will glide to a halt. In the case of the treadmill, the initial energy comes from the surface initially rotating the wheels. For open air, it's the wind pushing against the stationary surfaces of the car.

If you are incredulous, replicate the experiment- I sure want to. That's how science works.

[Charlie!]

After looking at both videos, I''m beginning to see how this is indeed possible.
First off, can we all see how a treadmill is equivalent to a wind across a flat surface? If not, ask for clarification.
As we begin, all of the device's components are stationary. As soon as it is placed upon the treadmill, the mass/inertia of the vehicle causes it to stay in pretty much the same place, at least for a bit. However, the wheels can still rotate freely underneath the car. Their rotation provides power to the propeller, which pushes the car forward. Already, it is moving "faster than the wind". Even then, as soon as it moves forward the speed difference between the car and the surface increases, providing yet more power to the prop.
It's not the propeller powering the wheels, it's the wheels powering the propeller. In fact, this problem is quite similar to POATM.
In neither case is there perpetual motion. As soon as the wind/treadmill stops, the car will glide to a halt. In the case of the treadmill, the initial energy comes from the surface initially rotating the wheels. For open air, it's the wind pushing against the stationary surfaces of the car.

If you are incredulous, replicate the experiment- I sure want to. That's how science works.

Power taken from body by wheels = X J/s
Power to propeller < X J/s
Power added to the body by propeller << X J/s

I think we can all agree on that, right? Nothing is perfectly efficient.

So when this thing is going faster than the wind, what's powering it? Well, the propeller. And what's powering the propeller? The wheels. And is the backwards force from the wheels larger or smaller than the forwards force from the propeller?

As long as the wheels are connected, this can never go faster than the wind. You really are asking the propeller to have more power going to it than is coming from the wheels. Ask yourself this: If this really worked, there would be some point where it accelerated constantly even if you took the wind away once it was there. Then why not hook it to a generator and get infinite free energy?

I, unfortunately, do not have the resources to replicate this. But I think that the fact that it violates conservation of energy is a pretty decent test.

[Rohit507]

again, at this point the question isn't if it's possible, it is. those videos and many more prove that, but how is it happening?

my big contention is, at a point the vehicle is effectively moving upwind, so why can't you just turn these devices around and see them go?

at the very least explain the ruler car? that makes just as much sense as a propeller power vehicle, but it's much simpler and much eaiser to replicate @ home.

edit: just tried it and it works as advertised. the thing nearly shot out from under my ruler.

i just realized the ruler car is symmetric, i suspect if you turn the propeller cars the other way around they will still go downwind faster then the wind

[Charlie!]

again, at this point the question isn't if it's possible, it is. those videos and many more prove that, but how is it happening?

my big contention is, at a point the vehicle is effectively moving upwind, so why can't you just turn these devices around and see them go?

at the very least explain the ruler car? that makes just as much sense as a propeller power vehicle, but it's much simpler and much eaiser to replicate @ home.

Nah, the ruler car is definitely the real deal, using good old gear ratiostotally didn't say that. The reason it makes more sense than the self-propelling car is because its speed is directly proportional to the ruler's and conservation of energy all works out with a little analysis. Would it be easier to visualize upside-down, with the ruler moving twice as far on the little wheels as the big wheel moves on the ground?

Also, if it's wrong, then it's quite definitely still a question if this is possible. And unless these guys show up in Nature next month on the front page (or someone just explains to me where the energy comes from) I will continue to claim that conservation of energy disqualifies them.

[Rohit507]

actually i tried it and it works no matter the size of the wheels, in fact having all 3 wheels the same size still lets it move faster.
the two systems are the same, if you place blades coming out from the ruler wheel, and have a baffle or something to stop wind turning it the wrong direction

[Diadem]

After looking at both videos, I''m beginning to see how this is indeed possible.
First off, can we all see how a treadmill is equivalent to a wind across a flat surface? If not, ask for clarification.
As we begin, all of the device's components are stationary. As soon as it is placed upon the treadmill, the mass/inertia of the vehicle causes it to stay in pretty much the same place, at least for a bit. However, the wheels can still rotate freely underneath the car. Their rotation provides power to the propeller, which pushes the car forward.

And where does the power the wheels are generating come from? What you're trying to do here is equivalent to trying to power a car by a dynamo attached the one of the wheels. It don't work.

It's not the propeller powering the wheels, it's the wheels powering the propeller. In fact, this problem is quite similar to POATM.
In neither case is there perpetual motion. As soon as the wind/treadmill stops, the car will glide to a halt. In the case of the treadmill, the initial energy comes from the surface initially rotating the wheels. For open air, it's the wind pushing against the stationary surfaces of the car.

For science' sake, draw a force diagram already. You'll see that this construction can not work. There's no net forward force.

If you are incredulous, replicate the experiment- I sure want to. That's how science works.

Not quite. Scientists usually don't spend their time and resources on experiments that they know are doomed to fail because they violate half a dozen laws of nature. But you can try it if you insist.

again, at this point the question isn't if it's possible, it is. those videos and many more prove that, but how is it happening?

Yes, why not ignore the half-dozen proofs I've given that it's impossible. Just assume that something that's impossible is possible and go from there.

at the very least explain the ruler car? that makes just as much sense as a propeller power vehicle, but it's much simpler and much eaiser to replicate @ home.

edit: just tried it and it works as advertised. the thing nearly shot out from under my ruler.

Yes, the ruler thing works. It works very well, in fact. But it's a totally different situation. The fact that the video claims it's similar doesn't make it so. As to why it works, I already explained that.

[doogly]

Gear ratios! Drat, I was hoping it was something oddball.

[Charlie!]

actually i tried it and it works no matter the size of the wheels, in fact having all 3 wheels the same size still lets it move faster.

Erm, pretend I didn't say that A little actual thought later and the gear ratio doesn't matter, since the distance of travel of the bottom wheels is identical to the distance of travel of the top wheel along the ruler.

But if you did make a version where gear ratios mattered I bet you could get that sucker going pretty fast

1) have one small wheel sandwiched between two large ones. 2) Have the ruler in contact with the small wheel and use the large ones to touch the wheels that are on the ground. 3) be sad when it doesn't work because of the different distance attempted to be traveled by the two sizes of wheels. 5) stop saying things that I later have to redact when they're wrong.

[Rohit507]

the two situations are functionally equivalent.

the orange lines on the second picture are fan blades.
the green arrow is wind
and the blue panels are baffles to prevent the wind from affecting the lower part of the fan wheel

(note you could design the mechanism much better, but this is the closest structurally to the ruler car and makes my point easier to see.)


also, i vote "for science's sake!" as an official atheist exclamation

[Diadem]

That all being said. I can make a vehicle that moves downwind faster than the wind. I can even make it move upwind with the same speed. It requires some creative interpretation of the rules though, and won't work in practise (but it will in theory, at least).

1) Build a craft with as little air resistence as possible
2) Put in an electromotor and a lot of batteries
3) Equip it with an really huge inflatable wind turbine
4) Tether it to the ground
5) Inflate your wind turbine and keep it running until your batteries are fully charged
6) Deflate your wind turbine and pack it really tightly in the back of your vehicle so it doesn't ruin your streamline
7) Remove your tethers and turn on your electromotor
Keep going until your run out of batteries
9) Repeat 4-8 until you reach your destination
10) ...
11) Profit!

In theory you can get an average velocity greater than windspeed this way. In practise, it'll never work; too many places where you loose energy. But it's a fun idea.

A car powered by an electromotor powered by a wind turbine is also usually not what people have in mind when they talk about 'powered by the wind'

[Rohit507]

diadem, you're overthinking it.

[Diadem]

the two situations are functionally equivalent.

the orange lines on the second picture are fan blades.
the green arrow is wind
and the blue panels are baffles to prevent the wind from affecting the lower part of the fan wheel

(note you could design the mechanism much better, but this is the closest structurally to the ruler car and makes my point easier to see.)

There's one problem with your picture though. Once your cart moves faster than the wind, the big green arrow in the right picture will point the other way. Since it will be pointing in the direction from which the wind is comming relative to your cart. For the ruler, this is not true. The ruler always pushes against the cart with the same force in the same direction, no matter how fast it's going. This is why the situations aren't comparable.

also, i vote "for science's sake!" as an official atheist exclamation

Thanks! I kind of like it myself as well

[Rohit507]

There's one problem with your picture though. Once your cart moves faster than the wind, the big green arrow in the right picture will point the other way. Since it will be pointing in the direction from which the wind is comming relative to your cart. For the ruler, this is not true.

assume the frame of reference of the car, and you'll see this isn't true. they both switch direction relative to the car. both exert a force at the top edge of the wheel. this is true for all frames of reference it's just much easier to see it if you pick the car as 0,0,0

also the motion of the wind as drawn is in reference to the ground. i'm not fixing it, too lazy

given that, still can't figure out why this is possible. (and it is, ignoring evidence is tantamount to faith, and it is much more likely that the evidence is true than it is not, given those who do the experiment seem to be moved to the "it's possible" side)

[Hawknc]

It's not the propeller powering the wheels, it's the wheels powering the propeller. In fact, this problem is quite similar to POATM.

First off, you're an evil SOB for mentioning that which shall not be named in this forum. Second, no, it isn't, because POATM assumes that the wheels are friction-removing devices rather than drivers. The wheels are a distraction and irrelevant in POATM, but they're very relevant here.

[Charlie!]

the two situations are functionally equivalent.

the orange lines on the second picture are fan blades.
the green arrow is wind
and the blue panels are baffles to prevent the wind from affecting the lower part of the fan wheel

(note you could design the mechanism much better, but this is the closest structurally to the ruler car and makes my point easier to see.)


also, i vote "for science's sake!" as an official atheist exclamation

They do look similar, but that doesn't mean they are necessarily the same. For example, what happens then you give the car under the ruler a push? It will move a bit and then stop, because it's driven only by relative motion of the ruler and the ground. What happens when you give the self-propelling car a push (assuming it works)? It will accelerate indefinitely. The reason I say that is because you claim that this will move faster than the wind in any wind, which is equivalent to moving upwind in any wind. But once it's moving it will observe even more wind that, assuming propellers work linearly (an approximation meaning 1 unit mo' wind = 1 unit mo' power) will be able to drive it even faster, faster and faster, as long as there's that little push at the start.

So why do I think that's impossible? Because in order for the wheels to exert a forward force, a backward force has to be exerted on the propeller. Let me say that again, louder. In order for the wheels to exert a forward force, a backward force has to be exerted on the propeller. Conservation of energy (or just a bit of math) says that these forces are, in fact, identical! No perpetual motion for you!

[Rohit507]

SInce the two systems are basically identical, your'e saying that once the ruler stops moving the car should keep on going? no there is energy coming from the movement of the ruler, one it stops the car stops, and the same goes for wind, once it stops the car will stop.

You're assuming inertia from the motion of the car makes wind to push the car, and if so should not inertia of the car relative to the ruler make it continue moving forward?

the driving force is the velocity of the ruler/wind relative to the GROUND, not the car.

someone who knows more physics than me, just draw a force diagram already.

[ThinkerEmeritus]

again, at this point the question isn't if it's possible, it is. those videos and many more prove that, but how is it happening?

I'm suspicious of the feasibility of this, but I don't know yet for sure. What I do know is that neither the video of the car in the wind nor the video of the treadmill prove the point. In the first, it is pretty clear that the wind is gusty, so there is no way to tell whether the cart is moving faster or slower than the average wind speed. In the treadmill video, the car is held against the treadmill until the turbine is up to speed, and it runs into the side of the treadmill pretty quickly. The turbine, once sped up, has kinetic energy, which can be used to keep the cart going for a while. The crucial point is whether it can keep going indefinitely, and the video doesn't give us enought information to tell.

Has anyone duplicated these experiments well enough to see if the process works long enough to guarantee that the process would work without using up energy stored in the turbine?

[Charlie!]

SInce the two systems are basically identical, your'e saying that once the ruler stops moving the car should keep on going? no there is energy coming from the movement of the ruler, one it stops the car stops, and the same goes for wind, once it stops the car will stop.

You're assuming inertia from the motion of the car makes wind to push the car, and if so should not inertia of the car relative to the ruler make it continue moving forward?

the driving force is the velocity of the ruler/wind relative to the GROUND, not the car.

No, I said they're NOT identical. There is energy coming from the movement of the ruler. You apparently have one of those wheelie things all made, so try it: attach a weight to the wheels somewhere and move the ruler, and you should feel more resistance. Yes, I'm saying that you assume that the speed of the air relative to the car makes the car go. I'm also saying that the motion of the ruler makes the wheels go. But the important bit is that what makes the wheels go is only the relative motion of the ground and the ruler, not that relative motion of the wheels and the ruler.

Didn't I say that in my last post? But anyhow, it seems we agree at least in that case.


So tell me, if the self-propelling car will go faster than the wind at any speed and is limited only by aerodynamics, what is the relationship between the wind speed relative to the ground and the speed of the car (edited because the other was complicated)? Does this work at any wind speed relative to the ground?

Edit: an easier but maybe less meaningful way to see the difference is as acceleration vs. speed. The wheels and ruler have one velocity (wheels relative to ground) dependent on another velocity (ruler relative to ground). The self-propelling car has its acceleration (the acceleration of the car) dependent on the relative velocity of the wind and the ground. However, now the velocities aren't dependent in the same way. Would you agree with that?

[Rohit507]

thinker, in the gusty video look @ the white flag on the craft.

also on the treadmill, the situation is that there is a treadmill moving <- and there is wind moving -> if the car moves forward on the treadmill it is moving faster than the wind, even if it goes off, assume an infinite treadmill and infinite wind of equal strength and direction, there's nowhere for it to run off and it keeps on chugging along, except for the fact that it ran off what would be different between the two situations, and what would be different between a 5 second run, and a 500 second run? you get no more information from the 500 second run.

@charlie:

sorry but your'e saying there is a difference between the two systems, and i don't see where it is. if one works the other should work.
the car should go faster than the wind/ruler as long as there is relative motion between the wind/ruler and the ground, and frictional forces aren't greater than the force available from the wind/ruler. the speed of the car = K*W where k is a positive constant determined by the gear ratio, and W is wind/ruler speed.

[Hawknc]

There is definitely a difference, though. One that immediately jumps to mind is that, in the ruler case, the ruler (wind) is driving the top gear (propeller), whereas on the vehicle, it's the propeller driving the vehicle into the wind. Assertions of it "obviously" working aside (Youtube videos aren't quite scientific proof), one very simple question needs to be answered for this to make sense: assuming the wheels drive the propeller to overcome drag, what drives the wheels? If you can answer this question, you've solved how this works.

[Charlie!]

sorry but your'e saying there is a difference between the two systems, and i don't see where it is. if one works the other should work.
the car should go faster than the wind/ruler as long as there is relative motion between the wind/ruler and the ground, and frictional forces aren't greater than the force available from the wind/ruler. the speed of the car = K*W where k is a positive constant determined by the gear ratio, and W is wind/ruler speed.

Edit: an easier but maybe less meaningful way to see the difference is as acceleration vs. speed. The wheels and ruler have one velocity (wheels relative to ground) dependent on another velocity (ruler relative to ground). The self-propelling car has its acceleration (the acceleration of the car) dependent on the relative velocity of the wind and the ground. However, now the velocities aren't dependent in the same way. Would you agree with that?

Also, did you read above what I said in large, bold letters? If not, let me say it again. In order for the wheels to exert a force forward, the propeller has to feel a force backward. Extremely easy math shows that these forces are identical. Seriously, try it. Start with a force on the propeller, and figure out how much force the wheels are feeling.

And... speed = k*W? So why did the car on the treadmill move so slowly and the car on the road move so quickly (they said it was about 1.5 times the wind speed)? If there were no air resistance on the body of the car, what would k be?

How do you feel about violating conservation of energy? If it could be shown that this thing violated conservation of energy, would you still think it was the real deal?

[Rohit507]

hmm?
i distintly said speed. i suspect that the speed of the car is relative to the speed of the air. not acceleration. you would have to accelerate up to the correct sped when the wind starts, but it's finally all dependent on speed.

so yes charlie, i agree, and i agree with what you said in bold, but there is still no difference between the systems, the propeller spins backward, as if it is pushing the car, and so does the top wheel in the ruler experiment.

[Charlie!]

hmm?
i distintly said speed. i suspect that the speed of the car is relative to the speed of the air. not acceleration. you would have to accelerate up to the correct sped when the wind starts, but it's finally all dependent on speed.

so yes charlie, i agree, and i agree with what you said in bold, but there is still no difference between the systems, the propeller spins backward, as if it is pushing the car, and so does the top wheel in the ruler experiment.

Have you tried calculating, even mentally, the force being applied by the wheels relative to the force hitting the propeller wheel? Because if you don't care then I think this discussion is pointless.


Either edit your post, or make a new post. Please don't do both with exactly the same block of text...

[Rohit507]

of course, the force exerted on the car by the wind is the same as the force exerted by the car on the ground, but the wind torques the main wheel, that torque is transfered to the drive wheel and friction between the drive wheels and the ground produces a normal that pushes the car forward.

the only way for everything to balance out is if the car moves in the same direction but faster than the wind. if it tried to move in the opposite direction to the wind (both directions relative to the ground) there would be no way that the numbers would add up.

to put it in other words, wind turns fan, fan turns wheel. now in the situation given the path of least resistance for the car is to move with the wind. If you mess with the gear ratios right you can get the car to move faster than the wind.

thanks, now i understand how and why it's happening completely, or reasonably so at least.

[ThinkerEmeritus]

thinker, in the gusty video look @ the white flag on the craft.

also on the treadmill, the situation is that there is a treadmill moving <- and there is wind moving -> if the car moves forward on the treadmill it is moving faster than the wind, even if it goes off, assume an infinite treadmill and infinite wind of equal strength and direction, there's nowhere for it to run off and it keeps on chugging along, except for the fact that it ran off what would be different between the two situations, and what would be different between a 5 second run, and a 500 second run? you get no more information from the 500 second run.

My point is that the car might not be able to keep the speed up. On the treadmill video, they spin up the turbine (lots of kinetic energy) and the car never goes anywhere close to the distance that I suspect the turbine's initial energy would carry it. In the gusty wind, sometimes the flag indicates the car is going faster than the wind and at other times it does not. I think that they have left open the possibility that the car's turbine is storing energy when the higher wind is pushing the car, and using that energy when the wind is low to move the car faster than the lowered wind. There were arguments given by people earlier in the thread that energy conservation forbids moving the car faster than the wind indefinitely. To prove experimentally that these arguments are wrong, the car must move long enough or constantly enough that any stored energy would be used up. I don't think it does.

My opinion is that it is possible to use energy stored in the turbine to move the car faster than the wind for a while, but that you can't maintain that speed indefinitely.

[Charlie!]

OH MY GOODNESS I SEE WHAT YOU MEAN. I still think it's wrong, but I see the analogy! You're saying that at some speed, everything meshes together like it does in the wheels and the ruler. The wind is flowing over the propeller at the same rate as the wheels are moving along the ground, right? This would allow you to make a device that would travel upwind, or with the right gear ratio then you could make one that "meshed" while moving faster than the wind, right? Unfortunately, this meshing, while being fun, would not provide thrust at any point. There's no immovable point of contact that will let you make the bottom wheels turn faster than the propeller. And that's the difference between the ruler and the wind.


Also, a note for the people in here NOT rohit , which I am blatantly stealing from Diadem because I liked it. This thing accelerates forward when put on a treadmill, as you all saw (note for a second that the device works by the same principles whether the wheels push the propeller or the propeller pushes the wheels, which should make you pause, but in this case it's got to be the propeller providing thrust, unlike in rohit's diagram earlier). So what's happening to the energy of the treadmill? It's going up, obviously, because the self-propelling car is exerting a force on the treadmill that's in the same direction as the movement of the treadmill.

But then, if the energy of the treadmill is going up, what's happening to the energy of the car? Well, it's going up too, since it's accelerating forwards, increasing its speed.

So what's happening to the energy of the air, the last component in the system? Well, it's going up too! The device pushes the air around in order to move forward, increasing the air's kinetic energy.

So if the total energy in the car/treadmill/air system is going up, where is the energy coming from?

[GBog]

Actually, I think this is possible. (And for the record, I've studied physics, if that gives me any authority.)

The threadmill might be the easiest way to show how it works. Assume that we tie the car to the frame of the threadmill, and let the threadmill drive the wheel, which again drives the propeller. Now what do we have? A rather strange electric fan. No violation of physics here. Now, this fan pushes air backwards, it exerts a force on the air. Per Newton's third law, there has to be a counterforce. That is, the air pushes the fan and what it is attached to. If the threadmill was standing on a frictionless surface, it would even move slowly forward. This would basically be like a hovercraft.

Forget the frictionless surface, and assume we untie the car. It is still pushing air backwards, and again, Newton's third dictates a counterforce. The car can't transfer much momentum to the threadmill via its wheels, so the car should move forward. Equivalence of reference frames dictates that the same should happen with wind and stationary ground.

So yeah, although I initially thought *HOAX*, after watching the videos and thinking for a bit, I think it is possible.

EDIT: OK, lots of posting while I was thinking and writing, [s]it seems as other people (rohit at least) have figured out how this works. Still leaving this though, as it might be a good explanation for people still doubting.

2nd EDIT: Sorry, rohit, I'm not really sure you've understood it afterall. You might have, but your explanation seems somehow flawed

[gmalivuk]

So what's happening to the energy of the treadmill? It's going up, obviously, because the self-propelling car is exerting a force on the treadmill that's in the same direction as the movement of the treadmill.

No, it's obviously not. Because the car isn't exerting any backward force in the wheels. In fact, it's exerting *forward* force with the wheels, because that's what's driving the propeller. So the energy of the treadmill is taken away by the car, and this gives energy to the car (moving its parts and moving it forward) and to the air (pushing it backward). But as soon as power is cut to the treadmill, it stops providing energy, and the whole thing slows to a halt.

No, tacking and beating have nothing to do with the resistance provided by the water....Here friction comes into play. By making the boat oblong instead of round, you can make it's friction in the y-direction huge, while keeping it's friction in the x-direction small.

That's the point I was making. You need to make the one direction of friction much higher than the other, in order to tack. If the resistance from the water was removed, the boat would mostly just go in the direction of the wind, and you certainly couldn't go against the wind at all.

The threadmill might be the easiest way to show how it works. Assume that we tie the car to the frame of the threadmill, and let the threadmill drive the wheel, which again drives the propeller. Now what do we have? A rather strange electric fan. No violation of physics here. Now, this fan pushes air backwards, it exerts a force on the air. Per Newton's third law, there has to be a counterforce. That is, the air pushes the fan and what it is attached to.

Right. The question then becomes whether it's possible for this force to be greater than the backward force exerted on the car by the treadmill (which is countered by the tie if we ignore the propeller to start with). So far, the only arguments I've seen on this point are pretty hand-wavy, and some don't even seem to know the directions of all the forces involved (see above), let alone the magnitude...

[Rohit507]

actually gbog, you've got it, the only thing that seems to be ignored is that the startup energy comes NOT from the fan turning but from the propeller pushing the whole car. http://www.youtube.com/watch?v=QTAd891IpRs&NR=1 in that video you see the fan start and the propeller turning to PUSH the car. it can't be just the fan turning the propeller, if so the propeller would move in the opposite direction. and the car would move backward.

The car takes energy from the air, and it moves the car towards that point where the system meshes, and takes energy to keep that speed. it doesn't accelerate.

in other news, i find this ( http://www.youtube.com/watch?v=IMEerIkO ... re=related ) design interesting.

also, i hope you guys understand i am ineloquent buffoon.

but here's my full explanation.

lets assume you have a car.
you connect the wheels to the propeller such that when the car moves forward, the prop pushes air backward.
(note this means it's adding momentum to the car in the forward direction)

the car in on a flat piece of ground with wind.

initially the wind pushes the car forward, with it pushing on ALL parts of the car.
this spins the fan to some of the energy goes from the wheel to the fan, which is added to the momentum of the car.

this takes energy from the air and adds it to the car.
the car now moving forward feels a frictional force with the ground, turning the wheels which then turn the fan.

right now fan pushes air <- and air is moving -> (note energy loss in the air) relative to both the car and the ground.
this happens till the speed of the car equals the speed of the wind.

the prop is still turning and the inertia of the prop provides enough energy to the car till the speed of the car is greater than that of the air.

at this point relative to the car, the prop is not pushing the air, as in the beginning, it is being pushed by the air. it is taking energy from the air and transferring it to the wheels. which are doing most of the pushing now.

the car keeps on taking energy and gaining speed till everything meshes. and the fan is spinning such that is neither adds nor removes energy from the air, and the wheels are exerting no net force (excepting that which the fan takes from the air to combat friction)

[Diadem]

Is this discussion still going on? Guys I already explained why it's not possible in the 2nd post to this thread. Is it really that hard to get?

Ok then once more. Imagine the car moving at exactly the velocity of the air. The air is stationary relative to the car. Therefore there is no exchange of forces between air and car. Can we agree on that? Good. What other forces are there. Well gravity pulling the cart against the ground of course, but that doesn't have a forwrd component. And then there's friction with the ground. Which is a backwards force. So there's no force at all pushing the car forward. So it's speed won't increase. It will in fact decrease due to friction. If you were the power the wheels, you'd be able to create a forward force. But what can power the wheels? An engine, sure, but we don't have one. And the propeller is stationary with respect to the wind, and thus not generating any force. It ain't even rotating.

Bottomline: Our car will slow down.

Now if this is not enough proof for you, let's look at the situation where the car is going faster than the windspeed. This is a tricker situation because there are more forces at work. Let's assume the car's velocity is constant, to make things easier. Let's look at the frame of reference of the car. So the car is stationary, and the wind is now moving with some velocity -v. Negative because it's in fact moving in the opposite direction as the car. Now look at the collision of an air molecule with the car. The amount of kinetic energy it has after the collision has to be smaller than what it had before the collision (it's transferring energy to the car after all). Which means the magnitude of its new velocity is smaller. But we don't know the direction yet. Now let's look at momentum. The air molecule hits the car and bounces off with a speed that is at most equal to its original speed, in some yet unknown direction. The momentum transfer is proportional to the difference in the velocities. But the original velocity is negative! So the monentum transfer can NEVER be positive. In the best case scenario it will be zero. But only if the new velocity of the air molecule is exactly the old one, in the same direction. Which only happens if the energy transferred to the car is zero. In other words: If the air molecule completely misses the car. Adding in friction with the ground (which means we require a positive transfer of energy from the air to the car) and we'll see that the car has to slow down.

Bottomline: Our car will slow down.

Please note that the ONLY two laws of nature I have used to proof this are conservation of energy and conservation of momentum. I didn't use anything else. And I didn't make any assumptions about the mechanism of propulsion, the nature of the wheels, the nature of (or indeed presence of) the frictionforce between car and surface. None whatsoever.

So either you reject conservation of energy, or you reject conservation of momentum, or you accept that a car can not continuesly drive downwind faster than the wind. There are no other options.

[gmalivuk]

Ok then once more. Imagine the car moving at exactly the velocity of the air. The air is stationary relative to the car. Therefore there is no exchange of forces between air and car. Can we agree on that? Good.

No, we can't, because there's a propeller there. Which is exerting a backward force on the air. If this is less than the backward force on the wheels by the ground, it slows down. If it's equal or greater, it can go faster than the wind. You have yet to explain why it *must* be less, because you seemingly have yet to understand how the device is actually alleged to work.

And the propeller is stationary with respect to the wind, and thus not generating any force. It ain't even rotating.

If the car is moving along the ground, then its wheels are rotating, and so of course the propeller is rotating. The propeller is driven by the wheels themselves. So while I'm still not convinced one way or another, your argument sure isn't helping, because you're still missing the whole crux of the issue.

[Rohit507]

gmalivuk, you're right as long as the car is still slower than the wind, as soon as it starts going faster the propeller will stop being the pusher and will be what takes energy from the wind. thereby pushing the car.

I made a huge edit to my previous post trying to explain everything.

also the whole point of a propeller is to make it so that one particular type of air/car molecule collision is prevalent and it's one that'll drive our car.