Physics Brainteasers

My question was about the vehicle in the video presented by ProtoTypical. Isn't it moving upwind, as opposed to the downwind direction of your brainteaser?--Percy

In the video in question, the vehicle is moving downwind (moving with the natural wind) as proposed in the brainteaser. You can confirm this for yourself at 0:20 in the video where there is a shot of an anemometer vane showing the wind blowing from right to left which is the same direction as the vehicle travels throughout the run..

Every good brainteaser much be tricky in the sense of non-intuitive. The tricky nature of this one is evidenced in the literally hundreds of thousands of posts on science / physics / etc. forums across the internet. I promise however there are no word games involved.

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Is it possible (theoretically or real world, either one) to build a wind powered vehicle which can travel directly downwind, faster than the wind, powered only by the wind, steady state?

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JBEdited by ThinAirDesigns, : No reason given.

Since you have two outside forces (tailwind first, then headwind) operating on the system, adding energy to the system, then perpetual motion it isn't. Initially, it is not the prop providing the power but the tailwind. The faster the wind moves the cart the faster the prop spins adding its function to the motion of the cart. The inertia of the cart allows it to pass through the transition to headwind. The headwind now helps spin the prop thus transfer power to the wheels. This cannot be kept up for too long since there is friction on the wheels and headwind pressure on the cart/prop and conservation of energy demands the cart/prop slow. The question is whether this setup can achieve a velocity faster than the initial tailwind. Not whether this setup can achieve and hold such a speed for eternity.[humor]As I've already mentioned, there are too many trees on this planet and eventually you will hit one.[/humor]Am I a sucker for a good story?Maybe I am.But, I'm not so sure.If I'm being a sucker, then I'm in good company.May I have a cookie? Please?Edited by AZPaul3, : No reason given.

I ran into this some time ago and had a debate with family about it. The initial reaction is that it is not possible, but when you look at the actual physics you can see that the answer is ...first: isolate the systems

1. system 1 is the wind
2. system 2 is the vehicle, which has subsystems
   1. the cart and
   2. a propeller driven by the wheels pointing backwards

The first impression is that 1 pushes 2 and the speeds approach v1, the speed of the wind (albeit with loss due to mechanical friction, vf, which is ~proportional to weight and thus is essentially constant once the system is moving) ...Replace the propeller with a disc and this becomes evident.... but this ignores that system 2 is pushing against system 1 by the propeller -- the faster the cart is pushed the faster the propeller revolves and the stronger the generated wind pushing aft, which would have a windspeed v2, approximately proportional to the cart speed, vc..Now consider a sheet (or disc) suspended by balloons free to move downwind via system 1 at air speed (v1), and then consider that system 2 pushes against the sheet from the other side generating additional wind speed aft (v2).So the force on the cart is v1*area + v2*area while the force on the sheet is only v1*area.-----------------------------------------------Technically the cart goes at vc = v1 + v2 - vf - vdWhere v1 is windspeed, v2 is propeller generated windspeed aft, vf is mechanical friction drag (relatively constant), and vd is "form drag" (proportional to area and to the "felt windspeed" squared), so vd ∝ (vc-v1)^{^2} and this approaches zero as the cart speed approaches windspeed and then picks up again as vc exceeds v1.and as soon as v2 > vf you can exceed v1 (vd → 0 at this point).Because vf is essentially constant while v2 is roughly proportional to vc (being driven by it) there is a threshold windspeed at which v2 = vf (when vd ≡ 0) -- you can see this point in the video when the windsock drops to hang straight downAnd then at higher windspeed vc > v1 ... and the windsock points aft instead of fwd.Once past v1 the apparent wind is a headwind, increasing as vc increases, and thus vd becomes a factor again, and as vd ∝ (vc-v1)^{^2} there is an upper limit to the speed that can be obtained.Clear as mud right? The action of the wheels against the ground is similar to the action of a keel (or iceblades) acting against a surface such that (land, water and ice) boats can travel faster than windspeed when traveling at 90°ree; to the (apparent) wind direction, which is another exercise.The friction loss is higher in water than on land, so it is harder for a boat than a cart.Enjoy.Edited by RAZD, : upper limit

As mentioned previously, this would violate the "steady state" clause in the brainteaser. Actually, the "steady state" clause means just exactly that -- that it could achieve and hold a speed faster than the wind for a theoretical eternity (given of course no trees to hit )JBEdited by ThinAirDesigns, : Typo

Into the headwind, no, can't be done. The cart will slow to the point that the friction of the headwind on the vehicle drops below the motive forces applied. Up to the speed of the tailwind and hold? No. Can't be done. No more motive force.You can, however, assuming the wind was right and constant, oscillate between these conditions until you finally hit that tree.When can I have my cookie? Edited by AZPaul3, : I hope we didn't pass messages before this edit.

I see the wind vane, but I was looking at the propeller. Isn't it spinning the wrong way if the wind is blowing from right to left?--Percy

I guess that would depend on which way you're expecting it to turn with the wind coming from right to left.Are you imagining the large spinning rotor to be operating as a turbine or a propeller? If you are imagining it as a turbine, then 'yes' it would be turning the wrong way for that to be happening.JB

The prop isn't free. It is driven, initially, by the wheels. The tailwind would drive the prop as a turbine. The wheels, however, keep it driven as a propeller.I think, I'll stop, not hog the thread and just watch and learn. I'll get my cookie maybe later.Edited by AZPaul3, : No reason given.

I'm just trying to follow the discussion. Proto asked if you meant "this sort of thing," and you said, "That certainly would be one attempt," but I don't see how it qualifies as being powered by the wind if the prop is being used as a propeller instead of a turbine.--Percy

Well, unless there is some internal power source and the vehicle has the capability for using stored energy to accelerate itself (which would be against the rules), any movement seen in that video (whether by turbine or prop) would be caused by the wind and thus the movement would be 'wind powered'.There have been plenty of accusations regarding that particular vehicle being internally powered and the video a hoax. Basically, one just has to decide if the OP question is possible or a violation of COE and if it IS possible, then a hoax is much less likely as internal power would not be needed.Also as a general note, I'm not trying to be coy in some of my answers in any attempt to be a a***ole, but rather to not simply give away the answer whilst multiple people are in the process of grokking it themselves.JBEdited by ThinAirDesigns, : No reason given.

Oh, okay. I assumed it was known how the thing worked.Why isn't it obvious on thermodynamic grounds alone that it isn't possible to sustain FTW travel on wind power only?--Percy

Some people DO claim to know how that vehicle works while others cry hoax. To a great many (including the physics professor linked to above) it is quite obvious, while others disagree.Let me summarize the argument for DDWFTtW -- while there is a CoE law, there is no such thing as a CoS (Conservation of Speed) law - thus the problem is an engineering challenge rather than a physics challenge. Simply put, the OP question boils down to this: can something travel faster than the medium which is propelling it?One video produced in an attempt to answer this follows: JB

Sort of feeding hints out here a bit at a time:People who insist that DDWFTTW (Direct Down Wind, Faster Than the Wind) is possible often use the treadmill examples to justify their belief. The notion being that Galilean relativity being what it is, a simple change in your frame of reference shows that a vehicle hovering stationary on a 10mph treadmill (in a still air room) is the same functionally as a vehicle on a natural road running 10mph directly downwind in a 10mph wind.The practical application of the above is that they insist that if the device can more forward on the threadmill (against the motion of the belt) it is demonstrating an instance of DDWFTTW.