Actually that seems wrong to me. In any scenario where the air is still relative to the ground, even where you’re rolling downhill, I believe that the extra resistance that the wheels would have in needing to power the propeller would exceed any thrust that the propeller generates and the propeller would not provide any excess speed.
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Absolutely maybe. I’m not entirely sure of myself.
But imagine the car is heavy, the hill is steeper and the propeller is very highly geared.
How does that seem?
In my example, the force of gravity is where the power comes from, instead of the wind.
I like your utilitarian approach.
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See –– we do agree!
Why would you suggest that would happen? How is it different with the a wind at your back? The wind at your back lowers the differential between the ground speed and air speed - it should be more effective, generate more thrust when there is no wind. It just could not get going without a push.
This is all decided by the physics - really no sense debating it. I am more interested in the human interpretation. The bet was won on the consideration that because the motion was started by the wind it qualifies as DDWFTTW. I say that was a generous interpretation because the action of the kart does not rely on the wind - it relies on the differential which still exists without wind. Its simply not a DDWFTTW machine. As such I maintain that DDWFTTW is impossible, just as the professor said.
You know about conservation of energy, right? Where do you imagine that the energy is coming from to keep it moving if there’s no wind?
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With no wind, the vehicle will just sit there, motionless.
So you push the vehicle to give it momentum and it will roll, the wheels will spin and they’ll drive the propeller. But the friction (tyres, chain drives, dragetc.) will quickly overcome the kinetic energy that your push invested in the vehicle and the vehicle will roll to a halt.
There needs to be energy coming from somewhere for the machine to drive.
This is why perpetual motion is impossible.
Does that make sense?
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Yes, that makes sense. But it makes me feel that you are not thinking about the relativity.
How does the wind at your back help you overcome that friction, etc once you have exceeded the relative speed of that wind?
I feel like you are saying that physics works for you when you want it to, and not when you don’t want it to.
The energy is coming from someplace. The propeller is being force into the air, because its moving faster than the wind. And because of its shape that relative speed acts like wind into a windmill and turns the propeller and transfers power to the transmission which gears it up to a faster than the wind land speed at the wheels.
Not having a tail wind works in favor of this action because the speed differential is greater. But it can’t get rolling by itself.
Does that make sense?
The energy comes from the initial push.
Tell me this - how does the wind supply energy, once the kart is moving faster than the wind. It can not. At that point the energy comes from the propeller breaking the air, and that forcing rotation, turn the gears, turn the wheels. This works better with no wind, as the differential is greater.
If you are saying it won’t work without wind - different than it won’t start without wind - please explain what the wind contributes once the kart is moving faster than the wind.
Initially, the wind just blows the vehicle downwind the way it would with a beachball. The wheels will roll and the prop will rotate but nothing very interesting will happen.
Now if the wind gets stronger and stronger, a threshold will be reached where the wheels spin the prop fast enough to exert some thrust and that thrust is operating on wind that is already moving. And at that point, the vehicle begins to travel at about same speed as the wind.
A little more wind and the wheels will go faster and the prop will make more thrust against the following wind, and the vehicle will travel faster downwind than the wind.
Its not perpetual motion. All of the energy is coming from the wind. Without getting into the mechanics of what is moving what, all the energy is from the wind. It is a wind powered vehicle and when the wind slows the vehicle will slow and when the wind stops. the vehicle will roll to a halt.
How does that seem?
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It seems obvious that once it passes the speed of the wind, and continues to accelerate, that it has transitioned to being powered by leveraging its own motion into propeller thrust. The wind can get it rolling, yes, but once the relative speed exceeds the wind, the wind can not contribute any energy.
Are you saying that the differential in aerodynamic drag between a tail wind and an no wind scenario is enough to bring the car to a halt?
Can’t wait to see the squirrel obstacle course he builds with that kind of money pouring in
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I think that’s what I’m saying.
It needs to be a good, strong wind. There is a threshold where the mechanism works and the stronger the wind, the bigger the difference in speed between the vehicle and the wind. (Up to a point where all your forces reach equilibrium again but I haven’t even begun to think about that)
If the wind starts to die off, the speed difference will lessen until the effect stops working.
I think thats what you are saying as well. However it was not mentioned in the video. Also the the model on the treadmill proceeds down the tread mill, creating its own headwind. Perhaps there is less mechanical and aerodynamic losses there.
But even with your premise accepted –– the wind speed removes drag, it does not add power. You still can not claim that this is being powered by the wind. You can say the wind is allowing the kart to overcome its mechanical and aerodynamic friction because the wind speed has reduced the friction the kart must overcome. A relativity relationship. But that is not the same thing as powering the kart. It still is not DDWFTTW.
Different Channels. This was Derek from Veritasium not Mark Rober from… the Mark Rober channel.
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NO WAY. I will bet you $10,000 that they are the same person!
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What your describing is perpetual motion, which is not a thing, and not at all what is claimed to be powering the blackbird. It is continually being pushed by the wind, without the differential in speed between the ground and the air (a.k.a. wind) it would not work. If you just gave it a push it would roll to a stop and not accelerate.
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FYI they had their best performance, as far as achieving the highest multiple of wind speed, when the wind was blowing between 10-12 mph. So not crazy strong.
Here’s a detailed account from an observer to the record attempt in 2010:
http://www.blueplanettimes.com/downwind-noir-the-record/
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Or as people called them throughout most of human history: “ships.”
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Sailboats don’t go 3x faster than the wind, (not to mention easily) and a physics professor was confused by the concept. (and our local thread grows)
Doesn’t seem cut or dry
