Comments on: Mythbusters — TONIGHT! http://www.variousandsundry.com/2008/01/30/mythbusters-tonight/ Wii, iPod, DVD, TV, and So Much More Mon, 01 Dec 2008 22:54:47 +0000 http://wordpress.org/?v=2.0.4 by: Scott Beeler http://www.variousandsundry.com/2008/01/30/mythbusters-tonight/#comment-148845 Thu, 31 Jan 2008 09:56:28 +0000 http://www.variousandsundry.com/2008/01/30/mythbusters-tonight/#comment-148845 Yeah, this is clearly not going to stop discussion/argument. What I wasn't expecting is why -- that the myth seems inadequately defined. From my point of view, they didn't even implement the myth properly. Their premise is that the plane "is moving at takeoff speed" on a belt moving at the same speed in the opposite direction, that the belt is "matching the plane's forward speed in reverse". To me this meant the plane is moving *relative to the belt* at takeoff speed, and so by definition the plane would seem stationary relative to someone standing just beside the belt. That's why I got so hung up in your earlier post in boggling over how a plane could possibly take off with no airflow over the wings. The answer is that instead of what I thought was the obvious meaning of the premise, they apparently meant that the plane's engine and prop are running at the rate that would under normal conditions be just sufficient to take off. Thanks to the fact that as you say the prop acts on the air, not the ground, this means that the plane will get up to the same takeoff speed relative to the air (and *twice takeoff speed* relative to the belt) and of course have the usual airflow over the wings and take off with no problem. It seems that if the wheels are sufficiently free-spinning (that there's sufficiently little friction) then what I thought was the premise is next to impossible to enact -- the belt will not be able to exert any force on the plane, so it'll refuse to stay stationary relative to a person next to the belt. So there's no question of take off or not take off, the situation basically just can't occur. I admit that I missed that when thinking about the myth. I guess I was figuring they could run the prop at a rate just fast enough to cancel that belt friction pulling it backwards and not any faster -- I didn't think the friction would be so tiny. But I was close to yelling at my TV by the end of the episode -- Jamie, if the plane moves forward off the end of the belt then the belt is clearly not "matching the plane's forward speed in reverse". :-) Now, for what it was, their implementation was fun and nicely done, but to me it seemed like a nice look at something different from what they said they were going to test. Clear wording is definitely a prerequisite for busting myths. :-) Yeah, this is clearly not going to stop discussion/argument. What I wasn’t expecting is why — that the myth seems inadequately defined. From my point of view, they didn’t even implement the myth properly.

Their premise is that the plane “is moving at takeoff speed” on a belt moving at the same speed in the opposite direction, that the belt is “matching the plane’s forward speed in reverse”. To me this meant the plane is moving *relative to the belt* at takeoff speed, and so by definition the plane would seem stationary relative to someone standing just beside the belt. That’s why I got so hung up in your earlier post in boggling over how a plane could possibly take off with no airflow over the wings.

The answer is that instead of what I thought was the obvious meaning of the premise, they apparently meant that the plane’s engine and prop are running at the rate that would under normal conditions be just sufficient to take off. Thanks to the fact that as you say the prop acts on the air, not the ground, this means that the plane will get up to the same takeoff speed relative to the air (and *twice takeoff speed* relative to the belt) and of course have the usual airflow over the wings and take off with no problem.

It seems that if the wheels are sufficiently free-spinning (that there’s sufficiently little friction) then what I thought was the premise is next to impossible to enact — the belt will not be able to exert any force on the plane, so it’ll refuse to stay stationary relative to a person next to the belt. So there’s no question of take off or not take off, the situation basically just can’t occur. I admit that I missed that when thinking about the myth. I guess I was figuring they could run the prop at a rate just fast enough to cancel that belt friction pulling it backwards and not any faster — I didn’t think the friction would be so tiny.

But I was close to yelling at my TV by the end of the episode — Jamie, if the plane moves forward off the end of the belt then the belt is clearly not “matching the plane’s forward speed in reverse”. :-) Now, for what it was, their implementation was fun and nicely done, but to me it seemed like a nice look at something different from what they said they were going to test. Clear wording is definitely a prerequisite for busting myths. :-)

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