EPT[SFW] Riddles / Puzzles / Brain Teasers - Page 28
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yB.TeH
Germany414 Posts
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mikell
Australia352 Posts
perhaps you should google how planes fly. | ||
Plexa
Aotearoa39261 Posts
On June 29 2012 18:31 mikell wrote: the plane won't take off, the air is not moving around the plane if the plane is not moving relative to the air. why do you think planes accelerate instead of just taking off nonsensically? | ||
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lpunatic
235 Posts
On June 29 2012 12:34 calderon wrote: if a plane was on a conveyor belt, trying to take off, but the conveyor belt would match the speed of the planes wheels PERFECTLY in the opposite direction, would the plane ever take off? ![[image loading]](http://msbrooks.com/blog/wp-content/uploads/2008/01/will_it_take_off.jpg) + Show Spoiler + No, but the conveyor belt will need to accelerate much faster than you think to satisfy the "match the speed PERFECTLY" condition, because you'll be spinning the wheels up with a couple of jet turbines' thrust worth of force Edit: Plexa, if you measure the speed of the "conveyor belts" the mythbusters use, you'll find they're always slower than the plane's wheels  | ||
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Plexa
Aotearoa39261 Posts
On June 29 2012 18:59 lpunatic wrote: + Show Spoiler + No, but the conveyor belt will need to accelerate much faster than you think to satisfy the "match the speed PERFECTLY" condition, because you'll be spinning the wheels up with a couple of jet turbines' thrust worth of force Edit: Plexa, if you measure the speed of the "conveyor belts" the mythbusters use, you'll find they're always slower than the plane's wheels + Show Spoiler + Yeah and its because the wheels just sit there. Theres no power going to the wheels to accelerate the plane, its all in the engines/propellers thrusting the plane forward. No matter how quick the conveyer belt is going, the plane will still go forward | ||
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lpunatic
235 Posts
On June 29 2012 19:03 Plexa wrote: + Show Spoiler + Yeah and its because the wheels just sit there. Theres no power going to the wheels to accelerate the plane, its all in the engines/propellers thrusting the plane forward. No matter how quick the conveyer belt is going, the plane will still go forward + Show Spoiler + You can, in theory, apply enough force to the wheels with the conveyor belt to keep the aeroplane in place, it just corresponds to an astronomically fast acceleration of the conveyor belt. There's no upper limit on how hard you can pull on the bottom of the wheels, but it's a bit misleading to talk about matching "speeds" instead of "forces" The wheels have mass, so there is a force associated with accelerating them - this is normally much smaller than the force associated with accelerating the aeroplane, but if you just set your imaginary conveyor belt to accelerate, say, 1000 times faster than the aeroplane would accelerate when it was unhindered, you could keep the aeroplane in place. | ||
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yB.TeH
Germany414 Posts
the plane accelerates through it's turbines wheels start spinning conveyor belt matches the speed of the spinning in the oppisite direction now the wheels are spinning twice as fast without slowing the plane down | ||
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lpunatic
235 Posts
On June 29 2012 19:09 lpunatic wrote: + Show Spoiler + You can, in theory, apply enough force to the wheels with the conveyor belt to keep the aeroplane in place, it just corresponds to an astronomically fast acceleration of the conveyor belt. There's no upper limit on how hard you can pull on the bottom of the wheels, but it's a bit misleading to talk about matching "speeds" instead of "forces" The wheels have mass, so there is a force associated with accelerating them - this is normally much smaller than the force associated with accelerating the aeroplane, but if you just set your imaginary conveyor belt to accelerate, say, 1000 times faster than the aeroplane would accelerate when it was unhindered, you could keep the aeroplane in place. + Show Spoiler + Actually, there will be a point where the conveyor belt will create enough wind for the plane to lift off, and this will probably occur faster than any upper limits on the wheel speed are approached - the only one I can think of is wind drag, which I imagine will always be insignificant compared to conveyor belt wind drag. It's a tricky question, the supposed "answer" disagrees with the conditions set by the question on the grounds that it's physically impossible (which it isn't, it's just harder to achieve than with a powered wheel type vehicle), but if you start disagreeing on account of physics then you have to ask just how much physics do you want to consider? | ||
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klicken
Sweden206 Posts
Yes it will take off. This is because the plane accelerates with its engines through the AIR, not with its wheels through the ground. You could compare it with a very aerodynamical car driving in heavy headwind, since the car accelerates through the ground, and due to its aerodynamical shape is isn't affected much by the wind and thus would drive just fine. | ||
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mikell
Australia352 Posts
On June 30 2012 00:13 klicken wrote: Oh my god, not the airplane again. Yes it will take off. This is because the plane accelerates with its engines through the AIR, not with its wheels through the ground. You could compare it with a very aerodynamical car driving in heavy headwind, since the car accelerates through the ground, and due to its aerodynamical shape is isn't affected much by the wind and thus would drive just fine. ??? the question states that the conveyor belt is moving at the exact same speed as the aeroplanes tyres. the aeroplanes tyres are the only thing propelling the plane forward. the THRUST from the ENGINES are TRANSLATED to the TYRES. if the TYRES are doing ZERO work then the PLANE is not TAKING OFF. LIFT is not generated unless the plane is moving at an appropriate speed. lift is not generated by the engines. the engines serve as a means to propel the plane forward. lift is created from the pressure difference between the top and bottom of the wings. this lift is not generated if the air is not moving over the wings of the plane (which would not be the case if the plane was on a conveyor belt and NOT moving relative to the air). i am not sure why this is so puzzling to some. and that mythbusters episode as above didn't have the conveyor belt moving at the same speed as the plane, if that was the case, the plane would not be moving relative to the observer at any point. | ||
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Phael
United States281 Posts
Here, the plane does not move relative to the wind (except the small breeze generated by the belt itself). Therefor, no lift is produced and the plane does not take off. | ||
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hzflank
United Kingdom2991 Posts
On June 30 2012 04:26 Phael wrote: A plane generates lift when air flows over its wings. Here, the plane does not move relative to the wind (except the small breeze generated by the belt itself). Therefor, no lift is produced and the plane does not take off. The plane does move relative to the wind. The plane moves forward at the same speed no matter whether it is on a runway or a conveyor belt. The wheels are not powered, so the conveyor belt simply makes the wheels spin twice as fast while the plane moves at the same speed through the air. The only way that a conveyor belt is going to stop a plane taking off is if it causes the whiles to spin so fast that the plane catches fire (or an equally stupid scenario). This is not rocket science  | ||
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Phael
United States281 Posts
In that picture, it's implied that the plane stays still on the conveyor belt - where its forward speed is matched by the belt. As in, does not move relative to a person sitting nearby. Yes, it only has the friction of the wheels to work on, but if the plane moves at all, even at a tiny speed, it will run off the edge of the treadmill rendering the scenario moot. So if it's moving forward at 100mph, the belt is also running at a high enough speed to push the airplane back, and their forces cancel each other out so the plane does not move. If there is no wind blowing, then the plane is also stationary relative to the wind. It's stationary to you, wind is stationary to you, commutative property, etc. In the mythbuster's scenario, the plane was NOT kept still. It was still moving relative to the viewer, and thus, the wind. So it was able to take off. | ||
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JinDesu
United States3990 Posts
A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane's speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off? A plane is standing on a runway that can move (some sort of band conveyer). The conveyer belt exactly matches the speed of the wheels at any given time, moving in the opposite direction of rotation. Can the plane take off? It all depends on the wording of the question. As long as the wording of the question does not specifically restrict the translational motion (i.e. conveyer belt exactly matches the speed of the wheels at any given time), then the plane will take off. This is true of the real world - there is no conveyer belt out there that can be made to prevent the plane from taking off. A conveyer belt or treadmill that can keep a plane with frictionless wheels from taking off will require to run to infinite speeds (constant force over time). | ||
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Golden Ghost
Netherlands1041 Posts
I mean in the mythbusters video the truck with Jaimy and the plane move in opposite directions and I can see how that would make the wheels move twice as fast. So what would the effect be if both the truck pulling the conveyor belt and the plane moved in the same direction? | ||
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hzflank
United Kingdom2991 Posts
On June 30 2012 04:51 Phael wrote: Ok maybe we are talking about two different things. In that picture, it's implied that the plane stays still on the conveyor belt - where its forward speed is matched by the belt. As in, does not move relative to a person sitting nearby. So if it's moving forward at 100mph, the belt is also running at 100mph, and their speeds cancel out. If there is no wind blowing, then the plane is also stationary relative to the wind. It's stationary to you, wind is stationary to you, commutative property, etc. In the mythbuster's scenario, the plane was NOT kept still. It was still moving relative to the viewer, and thus, the wind. So it was able to take off. You are completely missing the point. Because the plane's wheels are free (not powered), no matter how fast the conveyor belt moves, the plan will never be stationary relative to a person standing beside the conveyor belt. The planes engines cause it to accelerate through the air, not over the ground. It will always move forward. | ||
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Synche
United States1345 Posts
On June 30 2012 04:51 JinDesu wrote: http://www.mathrec.org/old/2010jan/solutions.html It all depends on the wording of the question. As long as the wording of the question does not specifically restrict the translational motion (i.e. conveyer belt exactly matches the speed of the wheels at any given time), then the plane will take off. This is true of the real world - there is no conveyer belt out there that can be made to prevent the plane from taking off. A conveyer belt or treadmill that can keep a plane with frictionless wheels from taking off will require to run to infinite speeds (constant force over time). It doesn't matter if the belt is going 1/2 the speed of the plane, or exactly, or twice. The air over the wings is completely independent of how fast the wheels are moving. Completely. The plane could be moving backward on a conveyor belt and it would still take off. | ||
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hzflank
United Kingdom2991 Posts
On June 30 2012 04:52 Golden Ghost wrote: And what would the effect be if you reversed the direction the conveyor belt is going? I mean a plane going forward has it's wheels spinning clockwise right? So if you had the conveyor belt turning in the direction the plane was going wouldn't that cancel out the movement of the wheels? I mean in the mythbusters video the truck with Jaimy and the plane move in opposite directions and I can see how that would make the wheels move twice as fast. So what would the effect be if both the truck pulling the conveyor belt and the plane moved in the same direction? Correct. If you reversed the treadmill then the plane's wheels would not turn at all...but it would still take off normally. | ||
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Phael
United States281 Posts
On June 30 2012 04:54 hzflank wrote: You are completely missing the point. Because the plane's wheels are free (not powered), no matter how fast the conveyor belt moves, the plan will never be stationary relative to a person standing beside the conveyor belt. The planes engines cause it to accelerate through the air, not over the ground. It will always move forward. This is only true in a frictionless scenario. In the real world (eg. that picture), a conveyor belt moving ANYTHING will exert a force on the object. Example: put a toy car on a tablecloth and slowly pull the tablecloth. The toy car moves relative to the observer. | ||
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hzflank
United Kingdom2991 Posts
On June 30 2012 04:56 Phael wrote: This is only true in a frictionless scenario. In the real world (eg. that picture), a conveyor belt moving ANYTHING will exert a force on the object. Hence my original post saying that the plane would catch fire before it was actually stopped. | ||
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