Questions about DDWFTTW

People follow their intuition and think that the dw component (for some reason) cannot exceed wind speed.

Thinking about tacking wasn't helping that much. As evident on this thread, I and many others wasted a lot of time on that issue. I suggest that next time someone doubts that this is possible, one can skip the whole discussion about tacking and go directly towards talking about the effective velocity between the air mass and the rotating, pitched blade in the sense of vectors (velocities), not scalars (speeds). It's the only thing that should really matter here.

We have found that there is no single explanation that works for everyone. Whenever someone finally "gets it" they usually ask why we wasted their time with all the other useless or even wrong explanations. But the fact is, we have a whole bunch of different explanations. They're all accurate, and each person seems to respond to a different one - and think the rest are nonsense.

Saying this means you deny that the cart involves the deflection of existing energy. That's like saying that no matter can be changing direction in this system. That has everything to do with blue and maroon arrows in the diagram below:



http://www.physicsforums.com/attachm...1&d=1325884076

Focus: Getting an Extra Bounce
Published October 4, 2004 | Phys. Rev. Focus 14, 14 (2004) | DOI: 10.1103/PhysRevFocus.14.14

Computer simulations and experiments show that a ball can rebound from a surface with more vertical speed than it had initially.
Anomalous Behavior of the Coefficient of Normal Restitution in Oblique Impact

Hiroto Kuninaka and Hisao Hayakawa
Phys. Rev. Lett. 93, 154301 (2004)
Published October 5, 2004

Figure 1
NASA-JPL



Quick sand. Computer simulations agree with experiments suggesting that a disk can hit a surface and rebound with a surprisingly vertical trajectory. Research on such impacts can help improve models of the flow of granular materials–such as these Martian sand dunes (shown in false color).

Figure 2
M. Louge/Cornell Univ.



Pop-up. In previous experiments [2] the trajectory of a ceramic ball hitting an elastic surface made a larger angle with the surface after impact than before. (See computer simulation video below.)
Animation courtesy of H. Kuninaka, Kyoto University.

http://physics.aps.org/assets/0d26d4...8/video-v1.ogg

High ball. This two-dimensional simulation shows how a ball can deform an elastic surface when it bounces. The surface becomes in effect a ski jump, which redirects the ball’s velocity skyward.

Like a gymnast who runs toward a vaulting horse and then hurls herself skyward, a ball can, under certain conditions, rebound from a glancing impact with a surprisingly vertical trajectory. It’s a phenomenon that’s been observed but never fully explained–and at times even doubted. But now researchers report in the 8 October PRL that they have developed a theory that explains the phenomenon and have tested it with computer simulations. Their explanation–which hinges on the ball’s impact deforming the surface it hits–could help refine models of the flow of granular materials such as sand dunes, cement, and soil.

The coefficient of normal restitution compares the vertical component of the velocity of an object before and after it has bounced. Conventional wisdom says it’s less than one–that is, a ball can’t leave the ground moving faster than when it arrived, because that would require extra energy (in the case of the gymnast, her body creates energy). But since the early 1990s several research groups have reported experiments with oblique impacts in which they found what seemed to be absurd results: A hockey-puck-like disk glanced against a wall and then appeared to pop away with an increased perpendicular velocity [1] and a ceramic sphere rebounded off a softer surface with a noticeably more vertical trajectory [2].

“They first thought I was crazy!” says Michel Louge of Cornell University, who performed the sphere-bouncing experiment with student Michael Adams. But after carefully ruling out experimental error, he concluded that the ball must deform the surface in such a way that it changes the trajectory of the ball. In that way, he thought, some of the horizontal component of the velocity could be transferred to the vertical component. He wasn’t sure exactly how this would happen, although it was clear that the effect was limited to special situations. “My conjecture in the [experimental] paper was just that–a conjecture,” he says.

Now Hiroto Kuninaka and Hisao Hayakawa of Kyoto University in Japan report that they have simulated the small-scale interactions between a disk and an elastic surface that can lead to a greater-than-one coefficient of normal restitution. Their computer simulation calculates a coefficient of 1.3 when the disk strikes the surface at an angle of about 11 degrees; at that angle, their simulated ball rebounds at about 15 degrees. The simulation results resemble Louge’s experimental data, according to the authors.

The simulation allowed the team to see the virtual disk denting the surface when it hit at oblique angles. Bill Stronge of Cambridge University in England describes the indentation as a kind of ski jump, which redirects the sphere’s velocity skyward. Because of this phenomenon, the coefficient of normal restitution can be greater than one without breaking the laws of physics. “The point is that the target material is softer than the ball,” says Kuninaka.

But Stronge doubts that the impact could make as vertical a ski jump as the researchers’ model suggests. It may have some effect, he says, “but I think that it’s certainly not nearly as dramatic as they have portrayed.” Computer models of granular materials such as cement and soil must account accurately for the collisions between grains, which can be treated like collisions with walls. So the work could contribute to practical advances in industries that manage and transport these materials, says Louge–and add to the understanding of the physics of ball sports.

–Chelsea Wald

References

J. Calsamiglia, S. W. Kennedy, A. Chatterjee, A. Ruina, and J. T. Jenkins, “Anomalous Frictional Behavior in Collisions of Thin Disks,” J. Appl. Mech.66, 146 (1999).
Michel Y. Louge and Michael E. Adams, “Anomalous behavior of normal kinematic restitution in the oblique impacts of a hard sphere on an elastoplastic plate,” Phys. Rev. E 65, 021303 (2002).

From this, we can see that "DDWFTTW" phenomenon isn't limited to sails or propellers

The very fact that these are "anomalous" tells us that they're not all that similar to DDWFTTW.

Bill Stronge of Cambridge University in England describes the indentation as a kind of ski jump, which redirects the sphere’s velocity skyward.

From this, we can see that "DDWFTTW" phenomenon isn't limited to sails or propellers:

Anomalous behavior of normal kinematic restitution in the oblique impacts of a hard sphere on an elastoplastic plate

This article has absolutely nothing to do with ddwfttw whatsoever. Your apparent desire to over complicate the simple lever which is the ddwfttw cart is fascinating to me. I believe your confusion stems from the fact that you still wish to see the cart as being pushed along by the wind. You need to think about the cart's prop exactly as you would the prop in a powered airplane. In this case the cart is powered by the wheels but it is generating thrust in exactly the same manner as the airplane.

"Powered by the wheels" makes no sense. They're not the source of energy.

The relative motion of the wind with respect to the ground is also not necessary, otherwise, dynamic soaring would not work.

And do you think that DD"W"FTT"W" can only happen with sails and propellers? I think just about any two interfaces will do. One of them doesn't have to be "wind". That's my point. I'm saying that DD"W"FTT"W" could done with anything, even with two "solids" if the angles are just right. DD"W"FTT"W" could also happen inside a fluid, where particles in the fluid can be likened to "ships" or "wind".

No, in fact the only thing that does make sense to say about the props power source is that it is the wheels. The ground is in fact the source of energy if you choose to analyze the cart in a frame other than that of the ground.

Dynamic soaring could also work in the upper atmosphere bordering the vacuum of space, by dipping in and out of it, but you can't give a vacuum a "velocity" with respect to the ground. So you don't even need to reduce the relative velocity of two masses to accelerate a third with respect to the first and the second.

This is completely wrong. The relative motion of the wind with respect to the ground is central to the carts functioning and the cart has nothing to do with dynamic soaring.

This is correct but completely contradicting what you said earlier about relative motion. As long as you have two surfaces which are in motion with respect to each other, you could design a vehicle which used leverage to travel faster than its power source.

I said:

Here, one air layer is "replaced with a vacuum", and I am saying that dynamic soaring will work, even with that.
So, the ground is not necessary. It "helps" but it is not necessary. So you would just need the blade and the wind.

I know what you said, its just that you are completely wrong. Your confusion seems to be fairly well entrenched at this point and I do not really know how to help you. At this point I will answer questions if you have any or hopefully others more expert than I am might chime in and help you understand.

...the cart has nothing to do with dynamic soaring.

So in other words, you think dynamic soaring cannot work between a vacuum (which lacks a velocity) and the upper atmosphere. Well, I'd like to see proof of that.

So in other words, you think dynamic soaring cannot work between a vacuum (which lacks a velocity) and the upper atmosphere. Well, I'd like to see proof of that.