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airplane vs conveyor belt

May 27, 2009, 1:03 p.m.
Posts: 239
Joined: March 18, 2008

i love this thread!

some awesome stuff (don't you guys take a lunch or anything) but still no "it will fly" people making this plane take off without having those goshdarn wheels spin faster the the conveyor.

Oh come on, I thought my infinite speed example was pretty good.

Flipping the onus onto the "it will not fly" people, how do wheels with no friction prevent the plane from taking off?

May 27, 2009, 1:09 p.m.
Posts: 5334
Joined: Feb. 3, 2006

As the plane starts to move forward, the wheel turns, and so the belt turns in the opposite direction to match the rotation speed. But… the wheel had to move in relation to the belt in order to rotate, so for that moment in time

May 27, 2009, 1:11 p.m.
Posts: 0
Joined: Oct. 17, 2008

ie: spin isn't speed.

badly worded. still the point is valid. if the plane takes off the wheels moved faster than the belt underneath them… which we all agree can't happen

Flipping the onus onto the "it will not fly" people, how do wheels with no friction prevent the plane from taking off?

the plane stationary. if you want it up the the sky you are gonna have to take off on the magic conveyor

this space is intentionally blank, other than this note about it being blank.

May 27, 2009, 1:15 p.m.
Posts: 239
Joined: March 18, 2008

the plane stationary. if you want it up the the sky you are gonna have to take off on the magic conveyor

But physically, what is holding the plane back against the thrust of the engines?

May 27, 2009, 1:22 p.m.
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Joined: Oct. 2, 2007

grins maniacally

May 27, 2009, 1:31 p.m.
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Joined: Dec. 12, 2007

But physically, what is holding the plane back against the thrust of the engines?

the magical conveyor belt system

May 27, 2009, 1:33 p.m.
Posts: 11540
Joined: June 29, 2006

The problem really is a test of understanding ground speed vs air speed and understand relative frames of reference for objects in motion.

:lol: The problem is just a definition of speed and where on the wheel we are talking about. From what I have read so far everyone has a pretty good handle of the physics. If you think the plane can move forward without breaking the rule (ie - measure speed at axle of wheels) then of course the plane can take off, because the wheels will just spin twice as fast as they normally would, but why wouldn't the question just say the "the speed of the plane" since it is the same thing. I see it more like a wheel trying to roll down an inclined treadmill where the treadmill speed is increased to keep the wheel from rolling downward. By matching speeds, the treadmill keeps the wheel locked in position, so the plane won't take off.

May 27, 2009, 1:37 p.m.
Posts: 557
Joined: May 27, 2009

Isnt this just a frame of reference issue as was stated before i think,

The wheel doesnt need to move faster then the belt.

The plane needs to move takeoff speed WRT ground.

For example, does it matter if you take off with or against the direction of the earths rotation ( or speed of ground WRT the earths center)… no

We can argue which frame of reference we are using till the cows come riding but it doesnt matter.

If the wheels can spin, the plane will move WRT the ground and we will fly????

EDIT: Never mind I think we all know this. I am going back to work

Don't be an engineer, every one of them I've met is socially retarded

May 27, 2009, 1:41 p.m.
Posts: 5334
Joined: Feb. 3, 2006

:lol: The problem is just a definition of speed and where on the wheel we are talking about. From what I have read so far everyone has a pretty good handle of the physics. If you think the plane can move forward without breaking the rule (ie - measure speed at axle of wheels) then of course the plane can take off, because the wheels will just spin twice as fast as they normally would, but why wouldn't the question just say the "the speed of the plane" since it is the same thing. I see it more like a wheel trying to roll down an inclined treadmill where the treadmill speed is increased to keep the wheel from rolling downward. By matching speeds, the treadmill keeps the wheel locked in position, so the plane won't take off.

^^ THIS!!!!!!!!!!!!!!!!!!!!! ^^

May 27, 2009, 1:41 p.m.
Posts: 239
Joined: March 18, 2008

EDIT: Never mind I think we all know this. I am going back to work

No no no, if we stop arguing then we have to go back to work and that's really boring.

May 27, 2009, 2:01 p.m.
Posts: 11680
Joined: Aug. 11, 2003

:lol: The problem is just a definition of speed and where on the wheel we are talking about. From what I have read so far everyone has a pretty good handle of the physics. If you think the plane can move forward without breaking the rule (ie - measure speed at axle of wheels) then of course the plane can take off, because the wheels will just spin twice as fast as they normally would, but why wouldn't the question just say the "the speed of the plane" since it is the same thing. I see it more like a wheel trying to roll down an inclined treadmill where the treadmill speed is increased to keep the wheel from rolling downward. By matching speeds, the treadmill keeps the wheel locked in position, so the plane won't take off.

Which was covered in my lengthy post above. The ambiguity of the question lies in the wording about the speed of the wheels. Angular velocity can be described by the surface velocity of the wheel (it's a function of the wheel's radius) and therefore, surface velocity can be related to the plane's velocity. Realistically, there are two points to measure the wheel's velocity from, the top or the bottom. The top yields twice that of the plane, the bottom yields 0 (Wheels roll, don't slide). Using the word speed removes the directional element of velocity, making the actual direction unknown, but under both criteria, there is no force to stop the plane.

I think what you are getting confused about, is that the plane is stationary, the wheels aren't moving, therefore the conveyor isn't, and the system is balanced, so you can't move the plane without upsetting that equilibrium.

May 27, 2009, 2:03 p.m.
Posts: 0
Joined: Nov. 19, 2002

:lol: The problem is just a definition of speed and where on the wheel we are talking about. From what I have read so far everyone has a pretty good handle of the physics. If you think the plane can move forward without breaking the rule (ie - measure speed at axle of wheels) then of course the plane can take off, because the wheels will just spin twice as fast as they normally would, but why wouldn't the question just say the "the speed of the plane" since it is the same thing. I see it more like a wheel trying to roll down an inclined treadmill where the treadmill speed is increased to keep the wheel from rolling downward. By matching speeds, the treadmill keeps the wheel locked in position, so the plane won't take off.

You always start out so well, then crash to the ground.

Please - all take note: [HTML_REMOVED]Gerewhore has been refusing to acknowledge my query (@12:00 today)[HTML_REMOVED]. I think he knows he's wrong, but just likes stirring up shit. And that's OK.

May 27, 2009, 2:05 p.m.
Posts: 0
Joined: Nov. 19, 2002

^^ THIS!!!!!!!!!!!!!!!!!!!!! ^^

….. is wrong!

You assumed that the wheels moved at all, if the Angular Velocity of the wheels is always equal to the Linear velocity of the Belt, wouldn't the wheels be unable to rotate at all and the plane rotate about it's fulcrum (wheel axle) and smash it's nose into the ground?

I got to admit. I can't figure out what you are saying. Willing to entertain the idea though if you want to re-phrase.

May 27, 2009, 2:23 p.m.
Posts: 239
Joined: March 18, 2008

here comes the FINAL ANSWER, end of thread.

note I've changed my position. the plane doesn't fly.

To answer the question you have to state your assumptions:
1. The speed of the wheel is the speed of the bottom (contact patch) relative to the plane. All other interpretations don't really make sense. This is the same as the angular speed of the wheel times the radius of the wheel.
2. The conveyor is magical and can go at any speed.
3. An ideal wheel has no friction, but has mass.

We know that the only way for the conveyor speed to match wheel is if the plane speed = 0 or infinity. People didn't like my infinity answer (which I thought was quite nice), so we will count that out as too abstract. So the plane has to stay still. That leaves the question of what is holding the plane back against the thrusters? The answer in the inertia of the wheels accelerating. Just like having heavy wheels on your bike gives you a backwards thrust as you try to accelerate.

So the plane sits still, but the conveyor keeps going faster and faster, the inertia of the accelerating wheels holds the plane back. The longer the plane tries to take off, the faster the wheels and conveyor will go until the plane runs out of fuel.

May 27, 2009, 2:28 p.m.
Posts: 0
Joined: Nov. 19, 2002

But.. won't relativistic effect some into play as the wheels approach the speed of light?
And isn't infinite speed [HTML_REMOVED] the speed of light? I think we'll have to invent some new science before we can allow that the wheels will actually travel faster than the speed of light.

[HTML_REMOVED]edit - objects approaching the speed of light increase in mass - so ignore the above - Still - the below is valid.[HTML_REMOVED]

You should change your position again.

You state you are making assumptions.. I don't think you need to make any assumptions - the question defines it all, if you play one statement against the other… and lose the idea of magic. If we're going to play with magic, then anything is possible, and all answers are relevant.

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