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09/16/2011 11:38:34 AM · #51 |
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09/16/2011 11:41:39 AM · #52 |
Originally posted by HawkinsT:
Yes, but it's a hypothetical question. So you interpret it as the conveyor originally matching the planes forward momentum but then the plane outruns it, I interpret the question as the conveyor can not be outrun by the plane. |
the sum of the external forces would be zero for the plane not to move, however the sum isnt zero and thus you get an acceleration. Forces can only be transmitted though certain connections and in this case the wheels are not one of those connections.
in the case you descried earlier where you pulled the plane backwards with the conveyor, you could only do that if the conveyor moved really slowly so that the friction force was transmitted though the bearings on the wheels and into the plane, once the wheels start spinning you lose that force transfer and the interia of the plane holds it in place. The turbines are tied directly to the plane so that it will always pull the plane forward, it doestt need to rely on a force transfer.
Message edited by author 2011-09-16 11:43:02. |
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09/16/2011 11:42:03 AM · #53 |
| Because it's a logic question, if you want a real world question then first you need more information, else I could add elements to your system that aren't defined by the question to either prove or disprove it. The only way you can give an absolute answer working only with the information provided is to treat is as a theoretical problem, not a practical one - in which case the plane does not take off. |
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09/16/2011 11:45:35 AM · #54 |
| mike_311: so going back to what I said already, you assume (since the wheels stay in contact with the conveyor until the plane has left the ground), that the wheels slip on the conveyor in order for the plane to gain forward momentum, OR that the conveyor can not keep up with the plane (nothing in the question states this is the case). Am I right here? And if so, then as I said, we are interpreting the question differently, that is where this point of contention is coming from. |
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09/16/2011 11:46:50 AM · #55 |
Originally posted by Cory:
Originally posted by mike_311:
OK now that that is solved, how about a new one:
Imagine you are sitting in a boat swimming in a pool. You have got a big rock in your hand. When you throw the rock into the pool what happens to the water level of the pool? |
Nothing, aside from some small waves. The displacement is equal, now if you throw it in from the shore there is an increased displacement.
Next? |
I
I've never seen a boat swimming in a pool. Got pix?
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09/16/2011 11:50:55 AM · #56 |
I think this is apt for the tread :)
There is no use trying, said Alice; one can't believe impossible things. I dare say you haven't had much practice, said the Queen. When I was your age, I always did it for half an hour a day. Why, sometimes I've believed as many as six impossible things before breakfast.
Lewis Carroll
Message edited by author 2011-09-16 11:51:55. |
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09/16/2011 11:55:06 AM · #57 |
Originally posted by HawkinsT:
mike_311: so going back to what I said already, you assume (since the wheels stay in contact with the conveyor until the plane has left the ground), that the wheels slip on the conveyor in order for the plane to gain forward momentum, OR that the conveyor can not keep up with the plane (nothing in the question states this is the case). Am I right here? And if so, then as I said, we are interpreting the question differently, that is where this point of contention is coming from. |
its not a case of the conveyor keeping up, it's the facts that the conveyor transmits little if any force through the wheels to the plane to counter the effect of the thrust. now if there weren't wheels....
the only case it wouldn't move its if you securely fastened the plane to the conveyor and which case the conveyors force could oppose the thrust equally and then conveyor wouldn't move.
in any other case, with skis, laying on its belly, ice skates, one of the forces is going to be greater, either the friction force from the conveyor or the thrust and one is always going to move the plane forward or backward.
Message edited by author 2011-09-16 11:59:57. |
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09/16/2011 11:57:29 AM · #58 |
The wheels of the plane are not what propels the plane forward so therefore the conveyer belt has no bearing on the planes motion. The propeller will pull the plane regardless if the ground beneath is moving or not.
Mythbusters.
Message edited by author 2011-09-16 11:59:13. |
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09/16/2011 11:57:33 AM · #59 |
Re Hawkins, If the wheels had anything to do with moving the airplane, other than allowing it to roll across the ground for take off and landing, airplanes would fall out of the sky shortly after take off. Thrust created by engines and propellers or by jet engines is what moves the aircraft, on the ground and in the air.
It wouldn't matter if the conveyor was moving at ten times the aircraft's ground speed, the aircraft would still gain air speed and be able to take off, provided that the tires don't explode due to the high rotational speed.
The only time that aircraft gain air speed by mechanical connection to the runway is on aircraft carriers.
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09/16/2011 12:20:30 PM · #60 |
Yes, you're all repeating a simple concept that believe me, I am fully aware of, the wheels DO NOT propel the plane forward... however the force required by the engines to move the plane forward is directly related (when the plane is static), to the resistance of the wheels assuming a level runway and engines exerting their force perpendicular to gravity.
For example, if the wheels don't slip and all wheels have a circumference of 1m, and the plane is moving forward at 1m/s, the plane in 1 second will have moved 1m forward and made one full rotation of its wheels. Now if you place a surface under the plane that is moving backwards at 1m/s, after 1 second the plane (retaining the same force as stated before), will have moved neither forwards nor backwards. You can increase the force generated by the planes engines as much as you like so long as you increase the force from the conveyor equally, and the plane will still not move. This is essentially the same as a rolling road, just a more awkward way to construct one. I rolling road built large enough for a plane would not allow it to take off. |
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09/16/2011 12:28:38 PM · #61 |
Actually I think this would be a good read for everyone, and then hopefully we can all move on =).
//blog.xkcd.com/2008/09/09/the-goddamn-airplane-on-the-goddamn-treadmill/ |
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09/16/2011 12:30:20 PM · #62 |
Originally posted by HawkinsT:
Yes, you're all repeating a simple concept that believe me, I am fully aware of, the wheels DO NOT propel the plane forward... however the force required by the engines to move the plane forward is directly related (when the plane is static), to the resistance of the wheels assuming a level runway and engines exerting their force perpendicular to gravity.
For example, if the wheels don't slip and all wheels have a circumference of 1m, and the plane is moving forward at 1m/s, the plane in 1 second will have moved 1m forward and made one full rotation of its wheels. Now if you place a surface under the plane that is moving backwards at 1m/s, after 1 second the plane (retaining the same force as stated before), will have moved neither forwards nor backwards. You can increase the force generated by the planes engines as much as you like so long as you increase the force from the conveyor equally, and the plane will still not move. This is essentially the same as a rolling road, just a more awkward way to construct one. I rolling road built large enough for a plane would not allow it to take off. |
not quite, you are still assuming the force is transferred through the wheel. in your example the force from the conveyor will cause the wheel to complete additional rotation but the force will NOT be exerted on the plane. while the plane still moves forward at 1 m/s from its own engines, while the wheels spin faster than they would normally due to the conveyor.
i keep harping on it but the wheels DO NOT transfer force. they spin freely. Cars move by exerting force on the wheels and the wheels exert a force on the ground, on a plane the spin free freely (at least during take off). |
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09/16/2011 12:31:45 PM · #63 |
Originally posted by HawkinsT:
You can increase the force generated by the planes engines as much as you like so long as you increase the force from the conveyor equally, and the plane will still not move. |
So... the conveyor has worked itself into a 1000mph frenzy, and the plane is still standing there wheels spinning like mad.
I stand behind the plane with a long metal rod and push really really hard. Is the conveyor belt capable of speeding up enough to generate enough friction to counteract that force? If I started pushing it, would appear to me that the plane is immovable? The rod will end up bending under the stress? |
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09/16/2011 12:32:21 PM · #64 |
Originally posted by HawkinsT:
This is essentially the same as a rolling road, just a more awkward way to construct one. I rolling road built large enough for a plane would not allow it to take off. |
No.
The difference is in where the force is applied. The "rolling road" the propulsive force is generated by the tires pushing on the conveyor. With a plane, the engine applies thrust to the air surrounding the plane, pushing it forward.
For planes, the rough equivalent to the "rolling road" example you cite for cars would be a wind tunnel.
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09/16/2011 12:35:32 PM · #65 |
lol! - nice link. Well, I did make sure and apologise in advance of starting this thread. It's also caused a few 'debates' here in the office today. |
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09/16/2011 12:45:30 PM · #66 |
there is a problem in this article.
you can't add Vb and Vw, the units are different, you need to convert one before you can add it.. one is a lateral velocity (distance over time) and the other is a centripetal velocity (rotations over time).
Message edited by author 2011-09-16 12:46:23. |
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09/16/2011 01:44:22 PM · #67 |
I read on another website that the Earth is flat. |
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09/16/2011 01:50:34 PM · #68 |
goddammit im not moving on, this whole link based on the assumption that the wheels can exert a force on the goddamn plane. The speed at which the wheels spin is irreverent, they exert no force in either direction!!! the wheels just spin and spin and spin, that's why they are F--ing wheels!!!
if the conveyor belt was moving twice, hell 50 times as fast as they needed to, the plane would still take the frig off!
until i see a proof that he conveyor belt gets force transmitted into the plane i will maintain my stance.
im making a new shirt, "The F---ing wheels are IRRELEVENT!!!"
Message edited by author 2011-09-16 13:54:53. |
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09/16/2011 02:15:19 PM · #69 |
Originally posted by mike_311:
... this whole link based on the assumption that the wheels can exert a force on the goddamn plane. The speed at which the wheels spin is irreverent, they exert no force in either direction!!! the wheels just spin and spin and spin, that's why they are F--ing wheels!!!
if the conveyor belt was moving twice, hell 50 times as fast as they needed to, the plane would still take the frig off!
until i see a proof that he conveyor belt gets force transmitted into the plane i will maintain my stance. |
haha, yes, but in the field of pure extravagant speculation, it's theoretically (though not practically) possible for the conveyor to accelerate infinitely fast--and thus for a wheel to rotate at such speed it will generate sufficient friction in the bearing (!) and/or heat fatigue the joint, or the centripetal force cause an explosion & so detain the plane.
But this said... the original forumlation specified the conveyor belt only rotating at the same forward speed as the plane. That speed won't come close to doing this...
Message edited by author 2011-09-16 14:33:05. |
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09/16/2011 02:25:09 PM · #70 |
Originally posted by Medoomi:
haha, yes, but in the field of pure extravagant speculation, it's theoretically (though not practically) possible for the conveyor to accelerate infinitely fast
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if you get infinite velocity, i get a friction free bearing :) |
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09/16/2011 02:31:00 PM · #71 |
Originally posted by mike_311:
Originally posted by Medoomi:
haha, yes, but in the field of pure extravagant speculation, it's theoretically (though not practically) possible for the conveyor to accelerate infinitely fast
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if you get infinite velocity, i get a friction free bearing :) |
yep, exactly--and I'd hate to calculate the speed anyway at which a normal bearing will heat sufficiently to actually cause some friction. Add in the lever arm of the wheel & to generate any meaningful resistance, it's got to be an astronomical speed... By which time the tire would have blown to smithereens...
Message edited by author 2011-09-16 14:33:33. |
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09/16/2011 03:31:01 PM · #72 |
Am I missing something here, or did Steve  Yo_Spiff already mention the fly in the ointment? No, the wheels don't move the plane, the engines do, but what makes it FLY is the lift from the wings. If the conveyor belt matches the groundspeed, then effectively there's no air moving across the wings. No air moving across the wings, no flight, right?
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09/16/2011 03:42:44 PM · #73 |
Originally posted by NikonJeb:
Am I missing something here, or did Steve Yo_Spiff already mention the fly in the ointment? No, the wheels don't move the plane, the engines do, but what makes it FLY is the lift from the wings. If the conveyor belt matches the groundspeed, then effectively there's no air moving across the wings. No air moving across the wings, no flight, right? |
Fans move air, conveyer belts do not. :)
So, if you placed a fan on the front of the plane which was able to push enough air to effectively counteract the planes movement through physical space, then you would have no lift from the wing surfaces, although, a powerful enough engine (think rocket, not jet) would provide sufficient thrust to make the aircraft fly, but the lack of airflow over the control surfaces would cause a total lack of control.
Message edited by author 2011-09-16 15:47:38. |
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09/16/2011 03:54:20 PM · #74 |
Originally posted by NikonJeb:
Am I missing something here, or did Steve Yo_Spiff already mention the fly in the ointment? No, the wheels don't move the plane, the engines do, but what makes it FLY is the lift from the wings. If the conveyor belt matches the groundspeed, then effectively there's no air moving across the wings. No air moving across the wings, no flight, right? |
Originally posted by Cory:
So, if you placed a fan on the front of the plane which was able to push enough air to effectively counteract the planes movement through physical space, then you would have no lift from the wing surfaces, |
The function of an airfoil is dependent on the airflow across it.
In your example, you *would* have lift.
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09/16/2011 03:55:56 PM · #75 |
Originally posted by NikonJeb:
Originally posted by NikonJeb:
Am I missing something here, or did Steve Yo_Spiff already mention the fly in the ointment? No, the wheels don't move the plane, the engines do, but what makes it FLY is the lift from the wings. If the conveyor belt matches the groundspeed, then effectively there's no air moving across the wings. No air moving across the wings, no flight, right? |
Originally posted by Cory:
So, if you placed a fan on the front of the plane which was able to push enough air to effectively counteract the planes movement through physical space, then you would have no lift from the wing surfaces, |
The function of an airfoil is dependent on the airflow across it.
In your example, you *would* have lift. |
From what? |
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