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| Newton's third law problem. |
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| electrodacus:
--- Quote from: Nominal Animal on November 22, 2022, 03:27:33 am ---And the physics view is that whenever there is wind with respect to the vehicle, energy can be extracted --- End quote --- Yes energy can be extracted but you are confusing energy with power. Power needed to drive against wind direction (upwind) is equal with the power wind already exert against the vehicle in the opposite direction. So if based on vehicle area and wind speed 100W act against the vehicle then vehicle needs 100W + friction losses to drive at any speed against the wind direction. So the only way to apply more than 100W is to store energy and then release it. Driving at say 30m/s with no wind will have the same power required to overcome drag as driving at 10m/s in a 20m/s head wind. So you agree with this above statement ? Because is not that is the the main disagreement that we have and this while not easy can be tested. |
| Circlotron:
--- Quote from: Nominal Animal on November 20, 2022, 07:00:58 pm ---A simplified example is a pair of wheels with almost as large a pulley in between, with a thread rotated on the pulley. What happens when you pull the thread? If you start examination at the zero-movement state, you'll find that the angle at which the thread exits the spool, with respect to gravity down, largely determines whether it starts moving or not. If you pull the thread exactly level, the forces do not leverage the wheel to turn so that it would travel opposite to the direction the thread is being pulled to. If you pull the thread slightly upwards or downwards from level, the forces are no longer balanced, resulting a small torque on the wheel, so it'll start traveling opposite to the direction the thread is being pulled to. If you pull the thread nearly vertical, the torque is maximized, and it is easiest to get the wheel to travel (away from where you are pulling the thread) --- End quote --- Underline added by me. I tried this experiment a while back with a roll of solder that had several layers used. The unused solder was about 5mm down from the cheeks of the plastic spool. With the solder extending parallel to the table top and coming from the underside of the roll you could pull the solder and it would roll up in a curious sort of way. It almost looked like an optical illusion. Something like you could pull the solder 20mm and the roll would move toward your hand 150mm, winding the solder back onto the roll as it went. Best results were when the extended solder was halfway between the two cheeks so the spool wouldn't pull to one side. A roll of solder is a good choice because it is reasonably heavy so the edges of the spool can have a grip on the table top without slipping, and everybody who's anybody has a roll of solder to try it with. |
| electrodacus:
--- Quote from: Circlotron on November 22, 2022, 04:32:11 am ---Something like you could pull the solder 20mm and the roll would move toward your hand 150mm, winding the solder back onto the roll as it went. Best results were when the extended solder was halfway between the two cheeks so the spool wouldn't pull to one side. --- End quote --- We are discussing about the opposite of your experiment. You pull the sting and the roll will go away from your hand. |
| Nominal Animal:
--- Quote from: bdunham7 on November 22, 2022, 03:56:19 am --- --- Quote from: Nominal Animal on November 22, 2022, 03:27:33 am ---And the physics view is that whenever there is wind with respect to the vehicle, energy can be extracted --- End quote --- Indeed that is what is apparent at first glance, but it isn't actually true, at least not if you mean with respect to the body of the vehicle. What is actually needed is simply any reference (the ground in the actual vehicle) and a wind relative to that reference. The point at which the wind speed equals the vehicle body speed isn't really that special at all because the action of the wind on the body is not the main event. --- End quote --- Sure. If you don't want to limit to any particular mechanism, then "wind with respect to ground" is better and more accurate. (I was thinking of devices without any energy storage, which are much more limited. My one-track mind and all. I now wish I had written "respect to the ground" instead.) Even a simple mechanism, say a VAWT (vertical axis wind turbine), which extracts energy from wind in any direction perpendicular to its axis, can be made much more interesting by using say three of them rotating around a common vertical axis. As long as the common axis rotates, all three experience a different relative wind speed and direction (relative to its own axis, that is) –– unless the wind is actually a vortex around that common vertical axis. (In this case the rotation around the common vertical axis can be considered energy storage, but it is a consequence of the mechanism.) Such mechanisms are very interesting to model and build, but I agree, it is not useful to discuss their theoretical properties in a forum like this: the signal to noise ratio is too low, with noise generated by well-meaning people basing their argument on intuition and experience with dissimilar devices, instead of physics. As to the simplest possible example of the pull-thread-thing-moves-the-other-way, just put a spool on an axis, and make two straight rails for the axis. If you pull the thread horizontally below the rails, the pulley will go in the opposite direction. Just make sure the axle and rails have sufficient friction, so that the axis rotates instead of sliding. With a heavy spool, the friction of the spool to the ground is so good that instead of sliding, pulling on the thread (or solder wire) so that it comes out from below, will actually cause the spool to rotate towards you, even when that causes the spool to rewind itself; and the axis of the spool will travel faster than you are pulling on the thread/solder. This is what Circlotron observed. |
| Nominal Animal:
And here is the simplest, most intuitive treadmill example I can think of, using Lego Technic. If you rename the attached treadmill.txt to treadmill.ldr, it too can be opened in LeoCAD. I omitted two rack pieces, and cut the long holey bar short, so you can see what is happening better. If you turn the blue handle clockwise, the upper surface of the treadmill chain goes right, but the gearwheel rotates counterclockwise and moves left. If you turn the blue handle counterclockwise, the upper surface of the treadmill chain goes left, and the gearwheel moves right, rotating clockwise. This is not a "locked gearbox", because the geared wheel can move horizontally. The racks and chain are not necessary for correct operation, they just provide lots of friction. The same works even with thread and smooth wheels, as long as you have sufficient friction that nothing slips. |
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