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A few weeks ago we reported on a fun and friendly wager between science communicator and creator of the Veritasium YouTube channel Derek Muller and UCLA professor of physics Alexander Kusenko. The point of contention between these two men was the mechanism behind a Blackbird, an experimental wind-powered land yacht. The puzzling thing is that the vehicle can move faster than the wind that propels it.
Muller released a video testing the blackbird and providing an explanation. But that did not sit well with Kusenko who got in touch to say that he disagreed with the explanation. After some back and forth, the bet was placed.
Kusenko explanation for the seemingly counterintuitive phenomena is that the wind changes. A strong gust of wind pushes the vehicle to a higher speed and then it calms down a bit so when the car speed is measured against the wind, the car goes faster but it is actually decelerating. Also, the wind speed at the height of the propeller might be stronger than the wind at the height at which is it measured.
“I’m excited about this bet because if I am wrong then I want to know,” Muller said in the follow-up video detailing the bet.“The whole point to the channel is to get to the truth!”
Muller set out to answer the point raised by Kusenko but also to provide a better explanation. Data from many tests have shown that the vehicle still moves faster than the wind, even at the height of the propeller and that when the Blackbird reached a record-breaking speed of 2.8 times the wind, it was still accelerating.
So how does it work? The propeller doesn’t work as most people think. Muller explains that it's not like a windmill, but more like a fan, pushing air backward. When the vehicle picks up speed, the wheels turn the propeller in such a way that it pushes the vehicle forward. Although friction slows downs the wheels, the push from the propeller is much greater, so the Blackbird moves forward.
Muller goes forward with the explanations showing how this approach also happens in other setups unrelated to wind-powered vehicles and he even got fellow YouTuber Xyla Foxlin to craft a model (several actually) that could demonstrate the downwind cart approach on a treadmill. And Foxlin’s cart did indeed work as expected accelerating faster than the treadmill.
“Professor Kusenko has now conceded the bet and he transferred $10,000 to me. I want to thank him for being a man of honor and changing his mind in light of the evidence presented,” Muller said in the YouTube video.
The money of the bet will now be awarded to other science communicators as part of a competition detailed on the Veritasium channel.
