The small demonstration that made it click for me is from that video:

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.

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

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.

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.

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/

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?

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.

Ok I have taken college level mechanics (after physics 1-2) and I have no idea what you’re saying.

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.

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.

I understand what is happening in the vehicle, I don’t understand what is going on with this explanation. We never learned about energy arbitrage when you “have different speeds.” We never learned anything about generating nearly infinite force against still air. These concepts sound really strange to me.

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.

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.

A propeller in air is fundamentally just like a roller on the ground, but with some efficiency loss.

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