5 ms·

The most surprising thing here is that someone on Twitter made a bet, lost the bet, and then actually paid said bet. Playing devil's advocate here a little: Th

The most surprising thing here is that someone on Twitter made a bet, lost the bet, and then actually paid said bet. Playing devil's advocate here a little: The whole machine is very counter-intuitive, and I can see how I myself may disbelieve it if not for the irrefutable demonstration(s)[0]. [0] https://www.youtube.com/watch?v=yCsgoLc_fzI https://www.youtube.com/watch?v=yCsgoLc_fzI

Tuna-Fish 5y ago
The small demonstration that made it click for me is from that video: https://www.youtube.com/watch?v=yCsgoLc_fzI&t=812s https://www.youtube.com/watch?v=yCsgoLc_fzI&t=812s This is exactly what the blackbird does, only with a propeller pushing air in place of the big wheel, and it's a much clearer and simpler demonstration of the same idea, which you can trivially verify.
- iamgopal 5y ago
  This is simplest and best explanation, no doubt why he got his PhD in physics education.
gibba999 5y ago
It's not at all counterintuitive. It's just poorly explained. Energy is 1/2mv^2, and momentum (related to force) is mv. Anytime you have different speeds, you can arbitrage. If I am pushing against still air, I can generate nearly-infinite force for arbitrarily little energy. That's why you use wings instead of pointing jet engines downwards. You push a lot of air by a little bit (lots of mv, not a lot of 1/2mv^2, since v is small). If I am going an epsilon faster than the wind, with very little energy, I can push hard on the air.
- ses1984 5y ago
  Ok I have taken college level mechanics (after physics 1-2) and I have no idea what you’re saying.
  - gibba999 5y ago
    Perhaps I can't explain it well either, without pictures. But it's not all too complicated. With pencil and paper, I could explain to a high school student. If you've done college level mechanics, that's on me, or on the medium. Perhaps I need to make a video explaining this clearly, since people are running in circles around this and it's just not very hard.
  - yodelshady 5y ago
    With due respect to you, and assuming you passed, that's pretty damning of whatever college you went to. I'd want a refund. The definitions of kinetic energy and momentum and some basic differentiation, that should all be thoroughly covered.
- whatsakandr 5y ago
  I think you're getting at it, but at the limit, you throw out the air, which Derek did in the follow up. Once you have two surfaces moving relative to each other, energy can be extracted. Doesn't matter how fast you're going relative to the two surfaces.
  - gibba999 5y ago
    I actually find the limit to be somewhat less insightful. It sorta shows that it's possible, but it doesn't really show why it works in this case. 1. Air isn't solid. 2. There aren't two rollers. The analogy is a little bit distant. I just need to find time to make mine more eloquent.
    - ummonk 5y ago
      A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.
      - gibba999 5y ago
        I understand that. However, that's not something I find easy to explain to a someone who doesn't understand that. The analogy is a little distant. For example, "some efficiency loss" is complex and a little bit inaccurate. It's not just efficiency loss: - In the mechanical analogy, I can extract infinite power from a board moving at a given speed. There is no upper bound on the force the board can apply to me. - In the case of a fan, there is an upper bound on the power which can be extracted given a specific air speed, even given perfect efficiency. Now, it's possible to correctly argue that as the fan size increases, I can make a closer and closer to this analogue. However: - Once you start making arguments like that, cognitive load increases, and beginners start to drop off. You need to keep more and more steps in your head. - And more often than not, an imprecise argument simply reinforces or introduces some misconception, rather than giving an honest understanding. I think a force*distance argument (which is probably a clearer way of stating the argument I was making above) has the virtues of: 1. Being 100% precise 2. Being compact enough someone who understands force, power, and energy can keep the whole argument in their head
ehsankia 5y ago
To be fair, they both agreed to make the whole bet public. They went on video, with "science celebrities" like Neil Degrasse Tyson and Bill Nye, and I believe they even signed something? It's kinda hard to backup from that. It also was a UCLA professor, so not quite some random person on Twitter. EDIT: As an aside, I would also add that the original video did a poor job at explaining, and the demo had clear flaws which were the points the physics professor was making. More specifically, his 2 main points were 1. It's possible that when the picture of the flag was taken, the wind was slower than the car, but the car was still moving faster due to momentum 2. It's possible that the wind speed up at the propeller level is different from the ground level where the flag was Those were both very valid criticisms, even though they didn't actually matter in the grander scheme of things.
notJim 5y ago
How does a professor have $10,000 sitting around for a bet like this?? I am a nicely-paid tech worker, and $10,000 wouldn't ruin me, but it would be a major setback for that year.
- ladberg 5y ago
  UC salaries are public[0] and it looks like he made $219k in 2019, which is pretty decent for a professor but yeah $10k would still definitely be a significant setback. [0] https://ucannualwage.ucop.edu/wage/ https://ucannualwage.ucop.edu/wage/
  - dave333 5y ago
    pretty decent for an incompetent professor in my opinion
    - rambojazz 5y ago
      oh c'mon now! Your insult is just outrageous. You're insulting a person who had the guts to make very valid questions and observations. Since when is questioning incompetence? Are you a man whose intuition is always right?

Tuna-Fish 5y ago

The small demonstration that made it click for me is from that video: https://www.youtube.com/watch?v=yCsgoLc_fzI&t=812s https://www.youtube.com/watch?v=yCsgoLc_fzI&t=812s This is exactly what the blackbird does, only with a propeller pushing air in place of the big wheel, and it's a much clearer and simpler demonstration of the same idea, which you can trivially verify.

iamgopal 5y ago
This is simplest and best explanation, no doubt why he got his PhD in physics education.

gibba999 5y ago

It's not at all counterintuitive. It's just poorly explained. Energy is 1/2mv^2, and momentum (related to force) is mv. Anytime you have different speeds, you can arbitrage. If I am pushing against still air, I can generate nearly-infinite force for arbitrarily little energy. That's why you use wings instead of pointing jet engines downwards. You push a lot of air by a little bit (lots of mv, not a lot of 1/2mv^2, since v is small). If I am going an epsilon faster than the wind, with very little energy, I can push hard on the air.

ses1984 5y ago
Ok I have taken college level mechanics (after physics 1-2) and I have no idea what you’re saying.
- gibba999 5y ago
  Perhaps I can't explain it well either, without pictures. But it's not all too complicated. With pencil and paper, I could explain to a high school student. If you've done college level mechanics, that's on me, or on the medium. Perhaps I need to make a video explaining this clearly, since people are running in circles around this and it's just not very hard.
- yodelshady 5y ago
  With due respect to you, and assuming you passed, that's pretty damning of whatever college you went to. I'd want a refund. The definitions of kinetic energy and momentum and some basic differentiation, that should all be thoroughly covered.
whatsakandr 5y ago
I think you're getting at it, but at the limit, you throw out the air, which Derek did in the follow up. Once you have two surfaces moving relative to each other, energy can be extracted. Doesn't matter how fast you're going relative to the two surfaces.
- gibba999 5y ago
  I actually find the limit to be somewhat less insightful. It sorta shows that it's possible, but it doesn't really show why it works in this case. 1. Air isn't solid. 2. There aren't two rollers. The analogy is a little bit distant. I just need to find time to make mine more eloquent.
  - ummonk 5y ago
    A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.
    - gibba999 5y ago
      I understand that. However, that's not something I find easy to explain to a someone who doesn't understand that. The analogy is a little distant. For example, "some efficiency loss" is complex and a little bit inaccurate. It's not just efficiency loss: - In the mechanical analogy, I can extract infinite power from a board moving at a given speed. There is no upper bound on the force the board can apply to me. - In the case of a fan, there is an upper bound on the power which can be extracted given a specific air speed, even given perfect efficiency. Now, it's possible to correctly argue that as the fan size increases, I can make a closer and closer to this analogue. However: - Once you start making arguments like that, cognitive load increases, and beginners start to drop off. You need to keep more and more steps in your head. - And more often than not, an imprecise argument simply reinforces or introduces some misconception, rather than giving an honest understanding. I think a force*distance argument (which is probably a clearer way of stating the argument I was making above) has the virtues of: 1. Being 100% precise 2. Being compact enough someone who understands force, power, and energy can keep the whole argument in their head

ehsankia 5y ago

To be fair, they both agreed to make the whole bet public. They went on video, with "science celebrities" like Neil Degrasse Tyson and Bill Nye, and I believe they even signed something? It's kinda hard to backup from that. It also was a UCLA professor, so not quite some random person on Twitter. EDIT: As an aside, I would also add that the original video did a poor job at explaining, and the demo had clear flaws which were the points the physics professor was making. More specifically, his 2 main points were 1. It's possible that when the picture of the flag was taken, the wind was slower than the car, but the car was still moving faster due to momentum 2. It's possible that the wind speed up at the propeller level is different from the ground level where the flag was Those were both very valid criticisms, even though they didn't actually matter in the grander scheme of things.

notJim 5y ago

How does a professor have $10,000 sitting around for a bet like this?? I am a nicely-paid tech worker, and $10,000 wouldn't ruin me, but it would be a major setback for that year.

ladberg 5y ago
UC salaries are public[0] and it looks like he made $219k in 2019, which is pretty decent for a professor but yeah $10k would still definitely be a significant setback. [0] https://ucannualwage.ucop.edu/wage/ https://ucannualwage.ucop.edu/wage/
- dave333 5y ago
  pretty decent for an incompetent professor in my opinion
  - rambojazz 5y ago
    oh c'mon now! Your insult is just outrageous. You're insulting a person who had the guts to make very valid questions and observations. Since when is questioning incompetence? Are you a man whose intuition is always right?