I don’t see what that has to do with (1). Will you grant me that for a car to move at 60mph as measured by a well calibrated and stationary radar gun that its wheels also have to move at 60mph according to a well calibrated speedometer?
The ball bearings don’t experience drag moving through the grease in the race of the bearing?
The only thing that affects the ability of the plane to take off is the airspeed. The speed of the wheels is irrelevant.
If the wheels and the treadmill match speeds (opposite directions) then the plane can’t be moving forward and therefore can’t take off (absent of a large headwind).
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Now I understand why that other board banned this discussion…
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Right, but how can the wheels and treadmill actually match speeds? The only way I can think of is reaching an equilibrium where the speed of the wheels is high enough to increase the friction in the wheel bearings enough to equal the force from the engines. For the speed of the wheels to match the speed of the treadmill then there has to be some force exactly cancelling out the thrust from the engine, right?
LFS said something similar and I get this part (I think). the challenge (in the context of the problem) is how that forward movement is happening on a surface that effectively is designed not to let that movement occur.
the answer as I understand it (from my reading of LFS’s earlier response), is that the engines at some point produce a thrust that is irrelevant (for lack of a better word) to placement on the plane on the treadmill at all. sort of like how you can stand on a skateboard on a treadmill but getting pushed in the back will lurch you forward and no increase in treadmill speed will compensate for it? the engine is inhaling and spitting out air, producing thrust, moving the plane forward relative to the ground regardless of the behavior of the wheels and treadmill?
Just watched Adam from Mythbusters’ explanation (10 min video) and I agree with him. I don’t get why people think anything weird is going on with the wheels but as he says, the argument will go on.
End of the demo episode is here (crappy video 3 min video but you get the idea):
Suzzer mentioned these earlier, can’t remember if he linked them.
A good troll might see what other arguments are banned in that forum and introduce them here at regular intervals.
A lot of the confusion and arguments around the airplane question stems from phrasing and interpretation of the question. I’ve seen two variants of this question both with different answers:
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The conveyor belt matches the ground speed of the plane. In this case the plane takes off no problem. The only thing the conveyor belt did was double the wheel speed of the plane.
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The conveyor belt matches the wheel speed of the plane. By wheel speed, I mean the speed that would be measured by a speedometer on the wheel. In this case, the plane doesn’t take off. Any non-zero ground speed makes it impossible for the wheel speed to match the belt speed. The constraint forbids applying any thrust at all, so the plane does nothing. This variant is more like a trick question than a physics question.
The MythBusters did the first question whereas Spider posted the second question.
There’s no way for the runway to impart enough force to the plane to stop its acceleration forward in this scenario.
Yes. But it’s important to see that a car is doing something different from what a plane is doing. If you imagine a car on a zero-friction ice sheet and a plane on a zero-friction ice sheet, the plane can move forward while the car will just spin its wheels in place.
Please make this stop
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That’s why poopypants is saying it’s a trick question. If the plane moves forward the wheel speed (if measured as a speedometer would measure it) would be higher than the conveyor belt speed, which violates the conditions of the problem. Poopypants’ answer to that is you can’t use the jets at all. Keeed’s answer is that the conveyor belt spins up so insanely fast that the friction internal to the wheel holds the plane in place.
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I guess I have to concede that the plane will not take off if we take the problem statement to mean that we cannot use the plane’s engines.
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That’s not what’s happening here. What’s happening is that you cannot move the plane forward without violating the conditions of the problem (assuming wheel speed is measured as a speedometer would measure it) (and as stated in this thread). The not using the engines is just poopy’s way of avoiding that violation.
See, a lot people would think that if the understanding of the problem leads you to that sort of absurdity, then one’s understanding of the problem must be incorrect.
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That’s silly. It’s a word problem someone wrote. It could either be a trick or it could be written badly. Understanding it as written though, again with an assumption about “speed of the wheels” which is vague, the plane moving forward violates the condition that the wheels and conveyor must move at the same speed.
In anything approaching the real world though, a plane clearly takes off. The forces acting on the plane are clear. But, if the wheels were hooked up to a speedometer that worked correctly on a stationary runway, it would end up showing a higher number than the speed of the conveyor.
Sure, the wording is vague, but there are basically two interpretations, one would be that it refers to the center of mass of the wheels relative to the ground, and another that states that problem definition is such that the plane can never move, it’s pretty obvious that the latter one is stupid. There’s no puzzle or physics or reasoning that goes into “Can a plane that cannot move take off?”