Down wind faster than the wind

[spork]

Marcus, if you want to watch the process that we've been describing, go the following link...

Indeed this is a physics professor that doesn't believe DDWFTTW is possible. But as far as I can see so far, he's also the most likely to be easily convinced. He hasn't started out by calling us idiots - which is a refreshing start.

 

[Dan O.]

A person traveling at 60kph, hitting a stationary car (B hits A) is another event. Totally different collision, and not a mirror of a 60kph car hitting a person. The kinetic transfer will be different. Timescales will be different. Impulse collisions are time defendant, and reversing the velocity vectors won't make them the same.

The person (possibly on a skate board) is traveling at 30 kph to the left with respect to the frame of reference R. The car is traveling at 30kph to the right with respect to R. From the results of the collision, how fast and in what direction is R moving with respect to the ground.

 

[ThinAirDesigns]

I wrote to said professor. Here is his reply;

"Sorry….I bowed out of that debate quite a while back. Let me assure you that what’s been attributed to me below (Sporks's quote) is not accurate. The discussion became far too heated, emotional, and folks weren’t listening to rational explanations."

'Nuff said

-
Not nearly 'Nuff said ... it was a three way phone call and two of those three are here on this thread.

spork:

He assures us that the treadmill bears no relation whatever to a road with a 10 mph tailwind.

100% correct. He must have told us this 20 different times and ways.

He also tells us that no test would ever satisfy him that our cart is faster than the wind.

I'll give that one only 99% correct -- I don't recall him saying that "no test would ever satisfy him", but no test that we could produce in a 20 minute phone call ever satisfied him and he was unable (even after repeated requests) to define one of his own for us.

He said even if we had our cart race a neutrally buoyant balloon on a direct downwind 5 mile course, and our cart reached the finish line in half the time of the balloon, that would still prove nothing.

100% correct. I asked the question about 5 different ways and got the same answer -- 'it proves nothing because the conditions aren't controlled enough.

He admitted that if an ice boat started and completed the course at the same time and place as our cart, it WOULD prove the ice boat is faster downwind than the balloon

100% correct. I first asked about the cart on the lake -- his response, "not proof". I then immediately asked him about an ice-boat on the same lake in the same wind -- his response 'proof, but that's different because of the skates'. (really, that's what he said). Somehow the "uncontrolled conditions" that weren't good enough for the cart were now somehow good enough for the ice boat.

...but it would NOT prove our cart was.

100% correct -- this was my question intended to get him to realize how dumb he sounded. I asked "so, you're standing on the far end of the lake, 5 miles from the start and theres a 5mph breeze blowing to you. A race starts between a freely drifting balloon, an ice-boat allowed to tack and a DDWFTTW cart constrainted to a straight downwind line. In 1/2 hour, both the ice-boat and the cart arrive at the EXACT SAME TIME and 1/2 hour before the balloon. You're telling me that they both arrive together and you check off on your clipboard "Proof" as to the ice-boat and "No Proof" on the cart?

He actually answered directly "Yes" and then went on to explain again the difference was the skates.

He assures us that 10 of his colleagues with vast experience in fluid dynamics have reviewed the problem and hold his same opinions.

100% correct.

JB

 

[mender]

Hold on, JB, don't convince everyone yet! You may need some dissenters for the Mythbusters segment! Just think of the entertainment value of some of the "theories" that have arisen.

I suspect that the prof's 10 colleagues were asked something like "can something get power from still air?", not giving them much of a chance of understanding the big picture. It does sound like he has an inkling about the kinematic restraint supplied by the skates but not how that relates to the wheels. Any chance of getting him on this forum?

 

[ThinAirDesigns]

I suspect that the prof's 10 colleagues were asked something like "can something get power from still air?", not giving them much of a chance of understanding the big picture.

Exactly.

It does sound like he has an inkling about the kinematic restraint supplied by the skates but not how that relates to the wheels.

Nope, and he wasn't about to listen either.

Any chance of getting him on this forum?

I doubt it -- he sounded on the phone like a deer in the headlights would look. He didn't what the heck he was talking about and didn't like questions that demonstrated that.

JB

 

[sol invictus]

Indeed this is a physics professor that doesn't believe DDWFTTW is possible. But as far as I can see so far, he's also the most likely to be easily convinced. He hasn't started out by calling us idiots - which is a refreshing start.

In fact, his last comment is:

s for the article, I made the point using a wind generator but I neglected any forces from friction. Obviously, this is a mistake. My main points are that energy has to come from somewhere. I will need to think about this some more. I know that fluid dynamics gets very messy, but I will try to grok the whole thing.

He realizes he's made a mistake, and I'm sure he'll figure it out.

ETA - added a comment to that blog that might help.

 

[Christian Klippel]

Well done. You might want to add a treadmill test to the video as well (very low speed, of course).

The "screw" version should be even more convincing, if someone can build one. The only place I can think of to get an appropriate screw would be from a post hole digger or ice auger.

I'll second the "Well done".

As for the screw thing, someone did it already. See here:
http://www.grogware.com/ddwfftw/

Greetings,

Chris

 

[Michael C]

Well done. You might want to add a treadmill test to the video as well (very low speed, of course).

The "screw" version should be even more convincing, if someone can build one. The only place I can think of to get an appropriate screw would be from a post hole digger or ice auger.

Thanks for the treadmill idea. I think I can simulate the treadmill by moving the piece of paper on the table while holding the ruler. Watch out for the next episode!

The screw version will be harder. I've had a look at the video that Christian linked to: I can see what's going on, but the video isn't clear enough to demonstrate the phenomenon to people who aren't quite sure how it works. I'm wondering where I can find a suitable screw, something lightweight that will work with my "tabletop" models.

 

[Modified]

I'm wondering where I can find a suitable screw, something lightweight that will work with my "tabletop" models.

Based on your latitude, you may have or know someone who has an ice auger. I think I used to have one that was fairly light plastic with steel only at the tips. That may not be quite small enough for a tabletop though.

 

[Christian Klippel]

I'm wondering where I can find a suitable screw, something lightweight that will work with my "tabletop" models.

Hello Michael,

what he used in the video looks pretty much like these anchors/screw used for hollow walls made from gypsum boards. It is basically a thread with a regular nut and washer for one side (the outside of the wall), and a butterfly-like mechanism for the other side (that goes into the wall). There is a "nut" in that butterfly thing to screw the thread through. You drill a rather big hole in the gypsum board, push the "collapsed" butterfly end through it, and when completely inside, it opens due to the spring inside. This will anchor the thread to the board then. They also come with a simpler mechanism.

It is called "Klappdübel" in german. Something like this here: http://www.mefa.de/de/_s/pdfs/products/catalog/anchor/hohlraum.pdf

Surely he used a high gearing in his model since the thread is obviously rather "fine".

Greetings,

Chris

 

[Modified]

Hello Michael,

what he used in the video looks pretty much like these anchors/screw used for hollow walls made from gypsum boards.

I'm thinking that a small screw like that won't really relate to a prop in peoples' minds, while something more auger-like would.

 

[Christian Klippel]

I'm thinking that a small screw like that won't really relate to a prop in peoples' minds, while something more auger-like would.

Hello Modified,

that's true indeed, haven't thought of that.

What about these spindles used for CNC machines, these ballscrews? While not really lightweight, they are readily available. Dunno down to what sizes these augers available, the ones i have seen with a quick googling look rather big. Also, a fitting "nut" would be needed, something that is definitely available for ballscrews. But then, ballscrew's are not really cheap, so it might be kind of overdosed for the purpose of a simple demonstration.

Greetings,

Chris

Edit: Michael C., your videos are really cool. I like your voice, it's really clear and nice to listen to. You really should do some audio-books for kids.

 

[Modified]

Also, a fitting "nut" would be needed

If big enough, you can just push it with your finger, or saw some grooves into a wooden stick. For something smaller than an ice auger, the auger from a juicer, pasta maker, or some other kitchen gadget would work.

 

[Christian Klippel]

Hmm, maybe a strip of acrylic glass can be used? Simply heat it up evenly and twist it until you get something auger-like? Should be easily doable over a kitchen stove, i think.

Greetings,

Chris

Edit: The twisting can be done by using a cordless screwdriver on one side, holding the strip with a caliper on the other side.

 

[John Freestone]

Jeez, I only took a day off and it's three pages longer!

Here is the explanation that worked for my sister:

Imagine you are a balloon. You are neutrally buoyant in the air that surrounds you (you neither float up or down).

<snip>

You end up building a cart similar to Spork's. Lowering it to the ground. Then, moving nearer to the object of your desire, and living happily ever after.

Lovely story, screensnot, and another good synapse tickler for those people trying to get this. I still struggle with the ground-power idea, and this does help.

I've put a new demonstration on YouTube. Inspired in particular by Modified and Brian M, I've made a simple mechanical cart that moves under the ruler faster than the ruler:
<video snipped>
The large wheel, turning against the direction of movement of the ruler, is very similar to the propeller of the wind-powered cart, turning against the wind.

Michael, that's not similar at all. Your ruler is acting as a springboard and the cottonreels are acting like interferometers and the real wind doesn't have little marks on it and the rubber on the big wheel is acting like a Van de Graaff generator that soaks up some of the electrical momentum of the table, although it does return some to keep things nicely in balance, but it's not doing what you think it's doing. Besides, your witnesses are only stuffed animals. You'll never do this in front of humans. (Sorry if you were having breakfast, JB)

A wonderful demo, Michael.

 

[Christian Klippel]

Michael, that's not similar at all. Your ruler is acting as a springboard and the cottonreels are acting like interferometers and the real wind doesn't have little marks on it and the rubber on the big wheel is acting like a Van de Graaff generator that soaks up some of the electrical momentum of the table, although it does return some to keep things nicely in balance, but it's not doing what you think it's doing. Besides, your witnesses are only stuffed animals. You'll never do this in front of humans. (Sorry if you were having breakfast, JB)

John,

you just made my day

Luckily there is quite a big time difference so its already late at night here in Germany. But it made me almost spill my beer. If i had one, that is.

Greetings,

Chris

Edit: It definitely must be the electron's momentum that makes it move, when they get ripped of their nucleus by the Van de Graaff generator!

 

[screensnot]

Lovely story, screensnot, and another good synapse tickler for those people trying to get this.

Thank you.

My 8th grade Creative Writing teacher once told me, "If your story has a happy ending, people will like it." (I was a bit rebellious at that age, and refused to let my main character live to the end of my stories)

 

[ThinAirDesigns]

Terry and the gang get a starring role.

http://www.boingboing.net/2008/12/12/downwind-faster-than-1.html

Congrats and very nice work Michael C.

JB

 

[spork]

... but the video isn't clear enough to demonstrate the phenomenon to people who aren't quite sure how it works.

And nothing on Earth will ever be clear enough to convince some of those that are just sure it can't work.

 

[Brian-M]

No, it cannot, "push back" in that sense. You could say, that it reacts to being accelerated, by "not accepting" the force, but that only puts you back to the same rate of acceleration that the calculations indicate.

Umm... NO. The propeller literally pushes the air backwards. That's why the propeller is geared to the wheels, so it can push the air backwards.

You pointed out my initial mistake, because the correct law says that your acceleration will be 1/4.

Only if you're assuming that the force applied to the cart is the same.

If the medium pushing it has a constant velocity, and the cart is applying force to push back on the medium (in order to push itself forwards), then the medium is applying more force to the cart then it otherwise would.

Not that the rate of acceleration has any bearing on the matter, just the terminal velocity.

I don't know how you are doing your calculations, but it is simple.
Do you agree, that the work is proportional to the amount of KE transferred to a given mass at a given velocity? E= 1/2mv2.
So 2V means 4E. That is the same for a 1:1 as a -1/2:1.
You can express that as differences in force, time, work, power or acceleration, but in the end it will be 4E. They are not my rules, Brian_M.

The amount of force applied to a regular cart by the wind diminishes to zero as it approaches wind speed.
The amount of force applied to a DDWFTTW diminishes to zero as it approaches (WindSpeed + BackwardSpeedOfPropeller).

Consequently, the DDWFTTW device recieves greater force from the wind over a longer distance than a regular cart.

As Work = Force x Distance, this means the DDWFTTW will have more kinetic energy than a regular cart.

If the carts are the same mass, and one has more kinetic energy, then it will be travelling faster. The wind supplies the cart with more kinetic energy than a 1/1 cart, which makes the cart go faster than a 1/1 cart. What's so confusing about that?

No, you are mixing up two ideas there. The question of the cart being driven by a conveyer, is different from that of a cart going faster than the driving medium, such as air.

Please explain how these two ideas are different.
If the cart being pushed by the overhead conveyer had poor traction between the top wheel and the belt, how would this be any different than the propeller cart? What forces would be different?