Think your smart?
- Thread starter Stangster5.0
- Start date
Guero said:
Going by the original question as stated by the thread starter, no way.
As the original question states - "The conveyor belt spins in reverse the exact same speed as the wheels at any given time"
But then in his follow up he says - "All that is gonna happen is the wheel speed will double"
So he really contradicts(sp) the original question to come up with his answer.
SWYZ721
Member
What creates lift is, in short, pressure equalization.
When the wings of an airplane cut through the air, the air moving over the top of the wing moves faster than the air going over the bottom of the wing. This creates a low and high pressure system, which wants to equalize.
If the wings do not move through the air (at all in this case) at certain speeds, not enough lift will be generated to pull the entire weight of the craft off the ground.
No birdy no flyey.
What we should gather from this, is that proper wording and attention to detail is paramount, proper communication is key, from instruction manuals (grrrrr) to saftey signs, we need to make sure our point comes across.
When the wings of an airplane cut through the air, the air moving over the top of the wing moves faster than the air going over the bottom of the wing. This creates a low and high pressure system, which wants to equalize.
If the wings do not move through the air (at all in this case) at certain speeds, not enough lift will be generated to pull the entire weight of the craft off the ground.
No birdy no flyey.
What we should gather from this, is that proper wording and attention to detail is paramount, proper communication is key, from instruction manuals (grrrrr) to saftey signs, we need to make sure our point comes across.
I still don't get how this question trips people up. The plane will fly just as though it were on a normal runway. Please read my previous posts for why.
But I'll try to revisit this again, maybe a little more straight forward this time.
The question says that the conveyor will spin backwards at the exact same speed as the wheels. SO WHAT. At the start, the plane is stationary, and so is the conveyor. Then we fire the engines. Equal an opposite reation, the plane moves forward. The only forces operating are the engines and the inertia of the plane. Now as the plane begins to move forward, the conveyor start moving, backward, at the same rate of acceleration as our plane moving forward. But what is the impact on the plane's forward progress? Exactly none. The wheels are not locked, the brakes are not on. The wheels merely spin faster. As the plane reaches takeoff speed, the conveyor is moving backward at the same speed as the wheels move forward (quite fast at this point), so our little wheels are spinning twice as fast as normal. Then . . . the plane takes off.
Yes there is forward motion, yes there is motion on the x axis, yes there is lift. The plane takes off.
The confusion comes from the thought that somehow the conveyor effects the plane. But it does not, at all. It's as if the plane were sitting on a perfectly normal runway. Does that help?
But I'll try to revisit this again, maybe a little more straight forward this time.
The question says that the conveyor will spin backwards at the exact same speed as the wheels. SO WHAT. At the start, the plane is stationary, and so is the conveyor. Then we fire the engines. Equal an opposite reation, the plane moves forward. The only forces operating are the engines and the inertia of the plane. Now as the plane begins to move forward, the conveyor start moving, backward, at the same rate of acceleration as our plane moving forward. But what is the impact on the plane's forward progress? Exactly none. The wheels are not locked, the brakes are not on. The wheels merely spin faster. As the plane reaches takeoff speed, the conveyor is moving backward at the same speed as the wheels move forward (quite fast at this point), so our little wheels are spinning twice as fast as normal. Then . . . the plane takes off.
Yes there is forward motion, yes there is motion on the x axis, yes there is lift. The plane takes off.
The confusion comes from the thought that somehow the conveyor effects the plane. But it does not, at all. It's as if the plane were sitting on a perfectly normal runway. Does that help?
SWYZ721
Member
GRGT1994 said:
It may not be possible, the wheels subject themselves to physics, the wheels and their bearings are designed to operate without exceeding a certain RPM. There is some leeway, but double? Maybe not.
Now that's if the question is understood correctly.
Confusion comes from the ill-wording of the original poster.
The engines creat ZERO LIFT, they create THRUST. Air passing over and under the wings create LIFT. Enough LIFT causes FLIGHT. FLIGHT causes POWWA. POWWA cause VTEC!!11!!!!!1one!!!1wun
yellow1995Cobra said:
The point is planes don't need wheels to take off, so the treadmill has no effect on it. Think about it like this, once the plane is in the air, what would happen if you spun the wheels backwards? Absolutely nothing. The wheels are only there to allow the plane to "float" in the air while it's landed. You can spin the wheels as fast as you want, but that's not going to slow the plane down unless the wheels lock up.
What about one of those planes that land on water? What would happen if it tried to take off in a very strong current going the opposite direction? The plane would take off as usual because it uses air to push it, not water. The propeller/jet engine is in the air, not the water.
A better question would be, would the plane be able to take off if the wind was blowing at the exact same speed as the plane in the exact same direction the plane is traveling?
blksn955.o
Founding Member
The question is not a logical one IMHO, making any answer ill-logical. The exact information is "based" on a set of questions that are not fully explained with the needed information on how it is setup to work.
I think the question is worded to do exactly this.
get people to devide into one camp or the other on the subject and those that are really smart will just keep banging away at the theory they subscribe to.
No one will build a working model to proove it one way or the other, yes math can prove it to a given extent but the "unknown" factors (like the whole treadmill workings setup), not to mention the weight alone.
I see the points made on the math side and the other side. This is my hangup though.
The biggest issue to get around for me is the wheel turning with the treadmill. As its worded it seems the wheels will move in the = but opposite to the treadmill and vise versa. If the plane is being thrusted forward and the treadmill goes backward at the same rate (as it says in the question) there is no movement. You add more force the treadmill will go more forcefull the opposite direction. That is if the wheels are fixed to the treadmill somehow to make sure the full force is put onto the treadmill and not overpowering the treadmill. This overpowering would be my real issue making the plane take off as movement would then start going forward but he question says nothing about if the treadmill can handle the forces.
Honestly, though I feel its a chicken before the egg issue the way its worded and not realy at the current time a realistic thing to even test.
I think the question is worded to do exactly this.
get people to devide into one camp or the other on the subject and those that are really smart will just keep banging away at the theory they subscribe to.
No one will build a working model to proove it one way or the other, yes math can prove it to a given extent but the "unknown" factors (like the whole treadmill workings setup), not to mention the weight alone.
I see the points made on the math side and the other side. This is my hangup though.
The biggest issue to get around for me is the wheel turning with the treadmill. As its worded it seems the wheels will move in the = but opposite to the treadmill and vise versa. If the plane is being thrusted forward and the treadmill goes backward at the same rate (as it says in the question) there is no movement. You add more force the treadmill will go more forcefull the opposite direction. That is if the wheels are fixed to the treadmill somehow to make sure the full force is put onto the treadmill and not overpowering the treadmill. This overpowering would be my real issue making the plane take off as movement would then start going forward but he question says nothing about if the treadmill can handle the forces.
Honestly, though I feel its a chicken before the egg issue the way its worded and not realy at the current time a realistic thing to even test.
I disagree. The question is worded fine. And there is a correct answer. But I think you hit the crux of the confusion here:
Now tell me this. How is the treadmill exerting any force on the plane? All we know it that it is setup so that it will move backwards at the same rate as the tires spin forward. Fine. And the tires merely spin faster. But still, no force transfers from the treadmill to the plane.
There is nothing in the question about the tires being affixed or attached to the tread mill. It mearly sits on top of it. We are not told by what mechanism the treadmill moves, only that it does. Who cares how it moves? There could be a million monkeys turning small cranks. Whatever. There still is no breaking force exerted on the plane. Engines fire, the plane moves forward.
blksn955.o said:
Now tell me this. How is the treadmill exerting any force on the plane? All we know it that it is setup so that it will move backwards at the same rate as the tires spin forward. Fine. And the tires merely spin faster. But still, no force transfers from the treadmill to the plane.
There is nothing in the question about the tires being affixed or attached to the tread mill. It mearly sits on top of it. We are not told by what mechanism the treadmill moves, only that it does. Who cares how it moves? There could be a million monkeys turning small cranks. Whatever. There still is no breaking force exerted on the plane. Engines fire, the plane moves forward.
So you guys are saying that the plane is moving forward absolutely (not spinning its proverbial wheels. Dont take that literally and post about engine thrust)? I still say that it isnt moving, given the wording and (perhaps just my) interpretation.
The only way the engine can make forward progress is via allowing the wheels to make absolute forward movement. And this would-be forward progress is negated by the treadmill/conveyor. So the plane is racing it's engine while at a relative stand-still.
I dont understand the spinning wheels in mid-air analogy because that is for a plane which is already flying (making absolute forward progress). I guess it seems to me that it's not possible for the plane in the original question to even begin to make forward progress.
The only way the engine can make forward progress is via allowing the wheels to make absolute forward movement. And this would-be forward progress is negated by the treadmill/conveyor. So the plane is racing it's engine while at a relative stand-still.
I dont understand the spinning wheels in mid-air analogy because that is for a plane which is already flying (making absolute forward progress). I guess it seems to me that it's not possible for the plane in the original question to even begin to make forward progress.
What tires could stop a plane from moving forward with its engines racing at full speed? If your Mustang can overwhelm the rear brakes and friction to spin its wheels, why don't you think a jet would overwhelm any force acting on its wheels? (not that I think there is any force acting on the wheels in this hypothetical, because there isn't).
This is more about logic than physics IMHO (becuase it's so hokey).
For every inch forward the plane's wheels rotate, the treadmill moves backwards to negate absolute forward movement. If the wheels cant make absolute X-axis forward movement, it's hard to generate lift.
I think we are interpreting something differently Brad (tough to imagine, given the concise wording of the original problem.
).
For every inch forward the plane's wheels rotate, the treadmill moves backwards to negate absolute forward movement. If the wheels cant make absolute X-axis forward movement, it's hard to generate lift.
I think we are interpreting something differently Brad (tough to imagine, given the concise wording of the original problem.
).HISSIN50 said:
I see what you're saying JT, but I don't think it works that way. My point with the mid air analogy is that the plane doesn't use the ground for movement. Look at Stangster5.0's theory about a bike on a treadmill. If you're off to the side, not on the treadmill, and you're pushing the bike, it's going to move forward no matter how fast the treadmill is spinning because an outside force is acting on it. The plane is using air to push itself along, not the earth(different elements
). The wheels are only there to hold the plane up in the air, so as long as they're turning and not locked up they're not going to affect the planes movement whether it's in the air or on the ground. What if a plane(probably need a helicoptor for this one) were to travel the same speed but opposite direction of earth's rotation, would it really be moving?
parchisi
New Member
Hi, I think I'll chime in. If you just want my answer, skip to the last paragraph, if you want my reasoning, keep reading. There are two ways of looking at this model. One way is to look at it from a stationary perspective, i.e. a fly on the wall next to the treadmill. From that view, an airplane taking off, the wheels would actually be turning faster than the treadmill is moving. But if you look at it from the perspective of the moving airplane, the treadmill starts to move faster in the opposite direction proportionally to increase of speed of the wheels. From the second point of view, the speed of the wheels and of the treadmill will remain equal and opposite.
The question, though, seems to be given from the fly-on-the-wall's point of view. For the airplane to make any forward progress, the tangential speed of the wheels would have to be faster than that of the treadmill. Take for example a wheel on normal, stationary tarmac. When the wheel is not moving, the speeds of the wheel and tarmac are equal in magnitude at zero. When you move the wheel, the speed of the wheel increases in magnitude while the tarmac still stays at zero. There you have a difference in speeds. The same idea still applies to the treadmill/airplane model. For the airplane to move through the air, the wheel's magnitude of speed must be greater than that of the treadmill. So theoretically, the airplane would not take off since the magnitudes of the speeds are equally opposite.
But the crux of the situation is that it is impossible to keep the speeds equally opposite. There is no way to make the treadmill go as fast as the wheels when the external force (engine) is applied and the airplane moves forward. If the treadmill did try to keep up, it would have to spin infinitely fast and the world would explode. (not really)
In short, from an outside perspective, the parameters of the question are physically impossible. From the perspective of the airplane, yes, the plane will take off. If I made a mistake anywhere please tell me.
The question, though, seems to be given from the fly-on-the-wall's point of view. For the airplane to make any forward progress, the tangential speed of the wheels would have to be faster than that of the treadmill. Take for example a wheel on normal, stationary tarmac. When the wheel is not moving, the speeds of the wheel and tarmac are equal in magnitude at zero. When you move the wheel, the speed of the wheel increases in magnitude while the tarmac still stays at zero. There you have a difference in speeds. The same idea still applies to the treadmill/airplane model. For the airplane to move through the air, the wheel's magnitude of speed must be greater than that of the treadmill. So theoretically, the airplane would not take off since the magnitudes of the speeds are equally opposite.
But the crux of the situation is that it is impossible to keep the speeds equally opposite. There is no way to make the treadmill go as fast as the wheels when the external force (engine) is applied and the airplane moves forward. If the treadmill did try to keep up, it would have to spin infinitely fast and the world would explode. (not really)
In short, from an outside perspective, the parameters of the question are physically impossible. From the perspective of the airplane, yes, the plane will take off. If I made a mistake anywhere please tell me.
pleasehelp
New Member
I think we need the mythbusters to solve this one. http://dsc.discovery.com/fansites/mythbusters/mythbusters.html
mo_dingo
20+ Year Stangneter 
HISSIN50 said:This is more about logic than physics IMHO (becuase it's so hokey).
For every inch forward the plane's wheels rotate, the treadmill moves backwards to negate absolute forward movement. If the wheels cant make absolute X-axis forward movement, it's hard to generate lift.
I think we are interpreting something differently Brad (tough to imagine, given the concise wording of the original problem.
I see where you are going with this JT. Here is where I think you are losing it..lol.
Ask yourself this. Why do the airplanes tires spin forward when it is accelerating? Why not backwards? Because the airplane's jets exerts a force in the positive X direction on the airplane itself, which makes the plane want to move forward. Because the ground is stationary, the friction between the tires and ground create drag. So something has to happen. The tires will not simply skid on the runway. They turn, in the direction of the least amount of resistance.
Now take our example. When you spin the conveyor belt from back to front, the ground is no longer stationary, thus the friction is severly less than with the stationary ground. If there is nothing for the plane to fight, the plane moves.
You know what I am thinking now, I actually think the plane will have a much easier time taking off, because the wheels are already spinning. Friction is much less once the object is moving. As you know, it's much easier to keep a box sliding on the ground than it is to initially start it.
The conveyor belt applies a force only to the wheels, which only applies a extremely small amount of force upon the airplane, again, due to the bearings lack of frictionless state.
The wheels can spin backwards, forwards, sideways, any way they want. The plane still accelerates and takes off.
Scott
I think this ^^ has been my line of thinking. As you all have seen, I've been focusing on the wheels, feeling that if the plane never moved forward with respect to perspective outside the plane, it wouldnt be able to take off.parchisi said:
Mike, what you commented about last is something I have a very hard visualizing - I can (or can't) see where you lost me.
I figured with an additional 'push', the bike wheels would accelerate, and the treadmill would accelerate in an opposing fashion with a result of no forward progress. Basically the treadmill's speed is a result of any and all forces acting upon the airplane/bike with respect to making the plane's/bike's wheels move forward. Like I said, this is hard for a simpleton like me to imagine in its dynamic.Scott, I understand what you are implying about static and kinetic coefficients of friction - I just dont understand how it applies to the problem. I dont understand what friction has to do with the problem, given the treadmill spinning at the same rate at all times as the tires. You also lost me.
As I think Brad said earlier, it doesnt matter how the treadmill turns - but it's a given that it has a force turning it in the opposite direction of the wheels at the same speed.
parchisi said:
The problem with this is that you're measuring accelleration by wheel speed. Your way is correct, but the plane does not accellerate with the wheels. It would still accellerate even if the wheels were not turning, although that would be a pretty nasty flat-spot
. You need to measure accelleration from the difference in speed of the plane itself and the ground. In this case the ground would be moving much faster than the plane, but that has no effect on the plane since it is essentially floating in the air and the outside force that propells it is not affected by ground speed, so we can assume the ground speed is zero. Even if the conveyor belt matches the wheels' accelleration, the plane will still be moving forward.JT, The bike tires and treadmill are both spinning at 30mph any you're standing beside the bike(on solid ground) holding it statioinary. You take a step forward, pushing the bike with you. The tires and treadmill increased to 31mph. How did they increase spead? You moved the bike forward. You only took one step so the bike is stationary again, with it's wheels spinning at 31mph now, but you had to move forward to get that increase in spead. If you pushed the wheel's themselves, then yes the treadmill would compensate for that and there would be no forward motion.
Another example, say you have a truck on solid ground with a rope tied on the rear bumper and the other end of that rope connected to the front bumper of a car on a conveyor belt spinning at 30mph, again matching the tire speed at all times. If you drive the truck forward, will the car not move forward with it? Or will the rope break because it's impossible for the car to achieve any forward motion?
Mike, thank you for the great analogies (esp. the towing one) - that makes perfect sense and it's something I can wrap my small brain around.
Like you pointed out, I couldn't get off the thought that X-axis displacement and wheel-speed/conveyor-speed and acceleration were directly related and cancelling, and that isn't the case.
Now in recalling what others posted, I can see where they were going with their thoughts and what they meant.
Thanks for taking the time to get that through my thick skull.
Like you pointed out, I couldn't get off the thought that X-axis displacement and wheel-speed/conveyor-speed and acceleration were directly related and cancelling, and that isn't the case.
Now in recalling what others posted, I can see where they were going with their thoughts and what they meant.
Thanks for taking the time to get that through my thick skull.
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