Comments by "seneca983" (@seneca983) on "Engineering with Rosie"
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@RogerGarrett "according to the builders, the propeller is allowed to freely rotate, with nothing other than the wind causing it to rotate"
No, that's not how it works and the builders aren't saying it is. The propeller is not allowed to spin freely and it spins to the opposite direction than the wind would turn it.
"And at that very point the wind is no longer imparting a force against Blackbird, . . . because both the wind and Blackbird are moving in the same direction and the same speed relative to the ground."
It's still imparting a force on the propeller because it's spinning.
"Then the propeller is engaged and begins to spin"
No, the propeller was engaged right from the beginning.
"The critical question then becomes, "What causes the propeller to rotate, where does the energy come from that powers the propeller?" It is NOT being caused to rotate BY THE WIND, because in the current state there is no wind relative to the propeller."
The wind does not directly rotate the propeller. Where the energy comes from depends on the inertial frame. In a frame tied to the ground it comes from the wind. In the frame tied to the air/vehicle (which are the same in this instance) it comes from the ground. Just because the wind doesn't directly spin the propeller doesn't mean the energy isn't coming from the wind, though that depends on the inertial frame used to look at the situation.
"They cite explicitly that the power comes from the wheels, from their interaction with the ground."
True. This in no way contradicts what I have said.
"It COULD come from a gasoline engine on board the vehicle. It COULD come from a flywheel on board the vehicle. It COULD come from solar panels powering an electric motor attached to the propeller."
Of course, I have not personally inspected the cart to know whether the builders have cheated in one of the ways you list. However, their record was certified by NALSA and trust them to have done a thorough enough inspection to know that no such energy sources are present.
"And when the vehicle is travelling at the same speed as the ground-relative air movement, that vehicle-relative air movement is ZERO and therefore contributes nothing, no energy, to the vehicle."
But in that frame of reference the ground is moving relative to the vehicle and that's where the energy comes from in that frame of reference. (In a frame tied to the ground the energy comes from the wind.)
"Further, when the vehicle is travelling FASTER than the ground-relative air movement, that vehicle-relative air movement is AGAINST the forward movement of the vehicle and therefore REDUCES the energy, the movement, the speed of the vehicle."
It doesn't because the propeller is spinning.
"Again, it is the repeated claim by the makers of the Blackbird that it is interaction of the wheels with the ground that powers the vehicle. That means that they are saying:
It is the motion of the vehicle itself that increases the motion of the vehicle.
It is the speed of the vehicle itself that increases the speed of the vehicle.
It is the momentum of the vehicle itself that increases the momentum of the vehicle.
None of that can be true. It is simply physically impossible."
It's not only the motion by itself. The interaction with the air and the ground is crucial. None of that is impossible.
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@RogerGarrett OK, here's my answer. (I'll consider the situation 2 only after my answers to 1 and 3, because that would distract from the main issue.)
Situation 1
Yes, I agree that in this case the cart would eventually stop. Even if the propeller is spinning and providing thrust it's being powered by the wheels and the grip of the wheels must be providing at least as much deceleration as the propeller is providing acceleration.
Situation 3
You said: "The only thing that the Blackbird experiences is the various forces imposed up on it by the Blackbird-relative air, which is ALWAYS a Blackbird-relative headwind, plus the standard friction losses."
However, those are not the only forces. If the Blackbird only interacted with the air it indeed could not exceed wind speed. However, it also interacts with the ground which is crucial. The wheels are being used to drive the propeller which means they are constantly braking the cart. However, since the ground is moving relative to the cart faster (100 mph in your example) than the relative headwind (which is only 50 mph in your example) it means the cart can get mechanical advantage in using the wheels to power the propeller. This means the wheels brake the cart less than what the propeller pushes it forward by. That means the cart can accelerate.
Situation 2
What about this situation? If you tried to use the same setup, i.e. the wheels powering the propeller, the cart would indeed stop even faster than in situation 1. However, you can do the reverse and use the propeller as a wind turbine and use it to power the wheels. This way you can again get mechanical advantage and accelerate beyond wind speed. Blackbird can operate like this as well and has gone upwind faster than the wind (but by a smaller factor than downwind)
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@RogerGarrett "The end of the shorter section of the beam moves as a result of the movement of the long section with the end of the shorter section moving through a shorter distance but with a higher force."
And that's also what's happening with Blackbird. The air is moving less relative to the cart than the ground.
"But then you look at the Propeller section of the vehicle and note that, relative to the ambient (Ground-relative) wind, it's going a much shorter distance (half the distance, in fact), and from that you calculate that there is TWICE the Energy there."
No, I calculated twice the force, not energy. I assumed the same energy, just like with an actual lever. Check again the numbers I used (though they're just made up examples).
Most of the rest of your comment seems to be based on this same misunderstanding. I never said the energy would be multiplied by mechanical advantage.
"The situation starts with figuring out how much Energy you can tap into from the momentum of the vehicle, from the speed at which the mass of the vehicle is travelling relative to the ground. That gives you a baseline, maximum amount of energy that's available."
No, that's hardly the maximum because the cart can extract energy from its surroundings (either the air or the ground depending on what inertial frame you're using).
"Leaves me wondering why you're not using that increased air velocity to calculate a LARGER distance travelled and thereby LESS Energy."
Because the propeller, if well designed, mostly pushes the air backwards. It doesn't accelerate to spin with it at high velocity. That would obviously be inefficient. The angle of attack of the propeller's blades are chosen such that these kinds of losses are not too large.
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@RogerGarrett "When you've got some object moving at some speed through the air it does not matter that the air may also be moving in the same direction of the object."
True, if the object is interacting only with that air. However, here the air is moving relative to the ground and the cart is interacting with both of them which is the reason why it's possible to extract energy from them.
"But if the propeller acquires its energy from the very momentum of the object itself that momentum will, must, reduce over time, with the result of the object slowing down."
But this isn't really an accurate description. You have tended to look at the situation in the inertial frame tied to the cart (at a specific instance). In this frame the ground is moving relative to the cart and therefore has kinetic energy. In this frame it's the kinetic energy of the ground which causes the wheels, and therefore the propeller, to spin. The ground then slows down by a minuscule amount. Thus the cart (and the air) are able to gain kinetic energy. (In a frame tied to the ground the energy would be coming from the wind instead but let's use the frame tied to the cart instead because you have tended to focus on it.)
It's like in your lever example. A lever doesn't multiply energy because the force and distance change by the same factor. However, you can still impart kinetic energy on an object through a lever. A similar thing is happening with Blackbird. The energy is coming out of the ground or the air (depending on frame) and the lever-like mechanical advantage is crucial for that being possible (but it doesn't multiply the energy extracted).
"And if the Blackbird can indeed extract energy from the air when that air is actually a headwind, imparting a drag force on it, then why doesn't that also apply to the situation where there is no wind, when the air is stationary relative to the ground?"
It's because of the mechanical advantage. If you remember my example with numbers. The air was travelling half the speed of the ground relative to the cart. Because of that the accelerating force from the propeller could be twice as large as the decelerating force from the wheels (assuming no losses). If, on the other hand, the air and the ground are travelling at equal speeds then there is no mechanical advantage. The accelerating and decelerating forces would be equal meaning zero acceleration and if you include losses or drag on the cart's frame then it would decelerate.
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@nc3826 ""Single use plastic disposable bags" was just a specific counterpoint example, of your generalized assertion that disposable products can be more environmentally sound."
I don't see how one example would in any way contradict what I said. I only said that for some products recycleability doesn't make sense, not all.
Also note that recycleability and durability are two different things. OP was only talking about recycleability, not durability, and my response was aimed at that too, not so much durability. FWIW, where I live plastic bags are recycled, at least if consumers put them in the right bin.
"Plus you could have given a different example, instead you mentioned, shoes?"
Shoes are a specific example of a product that would be too difficult to recycle once they are too worn out to be used anymore. How would you recycle them? It makes more sense to e.g. just burn them for energy than to recycle the material in them. (Sometimes burning is called "thermal recycling" but I don't think that comports with what people usually think of as recycling.)
"My true salient point, is the economic principles I gave earlier, based on negative externalities."
Yeah, negative externalities can exist. How big they are depends on the case. I'm just saying that in some cases they're not big enough to warrant recycling (or recycling could itself have too much negative externality). On the other hand, in some cases recycling does make sense. If you're worried about the negative externality of plastic bags, you can implement a Pigouvian tax on them like some places have.
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