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Post by BlueDolphin on Oct 13, 2007 2:01:19 GMT -5
I'm not completely sure where you guys are coming from here with the airplane being able to move as though the treadmill wasn't there, but the jet would be moving back through the air as well. The air doesn't move to match the jet's velocity. The engines would move it forward. It isn't just sitting there with its engines off. Once it started them, the plane would move forward relative to what it is pushing against (the air). Really? Once you lift your feet off the treadmill, I think it wouldn't be hard at all given that the treadmill (we are assuming) only moves the surface, not the water. As you have said, the air doesn't move to match your velocity. This analogy isn't that good since one would still have difficulty swimming with their feet dragging since there is a lot of friction from that. Wheels don't suffer from this as much though. What do you mean by this exactly? What's wrong with wheels? Try describing it rather than rejecting it as silly using vague terms. What it would give the wheels is energy to spin them backwards. And it doesn't really lose -1m/s. It depends on your frame of reference. From someone standing on the ground, the treadmill is spinning the wheels, but the wagon isn't moving. From someone standing on the surface, the wagon is moving forward at 1m/s. The hardest part of this is that there are two frames of reference. One that is stuck with the treadmill and the other that is on the ground.
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Post by Random on Oct 13, 2007 2:34:31 GMT -5
The engines would move it forward. It isn't just sitting there with its engines off. Once it started them, the plane would move forward relative to what it is pushing against (the air). And then the treadmill would increase its speed to match the jet's forward movement and continue to keep it stationary relative to the ground and air, still resulting in 0 lift. Really? Once you lift your feet off the treadmill, I think it wouldn't be hard at all given that the treadmill (we are assuming) only moves the surface, not the water. As you have said, the air doesn't move to match your velocity. This analogy isn't that good since one would still have difficulty swimming with their feet dragging since there is a lot of friction from that. Wheels don't suffer from this as much though. With the example of a jet, to lift itself off of the treadmill it would need lift, which would require it to be moving forward at fairly fast speeds, which is impossible because its being kept stationary by the treadmill. To leave the treadmill the jet has to leave the treadmill, and it can't. Wheels are not AT ALL different. Take a hotwheels car or something to a grocery store and put it on the conveyor belt. Its wheels aren't going to move just because the conveyor belt is. What do you mean by this exactly? What's wrong with wheels? Try describing it rather than rejecting it as silly using vague terms. What it would give the wheels is energy to spin them backwards. And it doesn't really lose -1m/s. It depends on your frame of reference. From someone standing on the ground, the treadmill is spinning the wheels, but the wagon isn't moving. From someone standing on the surface, the wagon is moving forward at 1m/s. The hardest part of this is that there are two frames of reference. One that is stuck with the treadmill and the other that is on the ground. Yet again I don't know the term but thats another problem. There are no two frames of reference unless you're silly enough to consider a person on the treadmill who isn't resisting its movement what everything else is moving relative to. Someone standing on the treadmill would be experiencing their own negative velocity relative to the ground, not the wagon's movement. Similar to assuming the sun rotates around the earth because you see it pass overhead every day, when in reality we're just spinning and orbiting the sun. Oh and if you move to the perspective of a person on the treadmill with the wagon you can't really count the negitive velocity from the treadmill and then say the wagon is moving 1 m/s forward because to him that -1 m/s becomes 0 m/s.
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Post by BlueDolphin on Oct 13, 2007 3:21:06 GMT -5
And then the treadmill would increase its speed to match the jet's forward movement and continue to keep it stationary relative to the ground and air, still resulting in 0 lift. That's the trick. Can you do that? It will try to match the jets movement, but since the jet is going to move forward relative to the air, the treadmill spinning underneath will not be able to keep it stationary. Can you think of a treadmill spinning so fast that a person standing on the ground can't pull the wagon forward? If you pull it towards you, then it moves towards you. The wheels will just spin faster to compensate. I was saying that your swimming example isn't very good since frictional force from your feet drags you backwards and makes it hard to swim forward (assuming that one even needs to get a running start, which is false since we float). Wheels are in fact quite different from dragging your feet. There is a reason that cars have wheels and not sleds because sleds have a lot more friction. I'm not sure what you are getting at with the hot wheels car. If you move the car forward at a certain speed, you will always be able to do so. If you push the car forwards relative to the ground at 1m/s, then spin the conveyor belt backward at 1m/s, the car will move forwards relative to the ground at 1m/s, not stand still. Can you describe what it was about? I'm curious to know what it is. To someone standing on the treadmill, the wagon is certainly not moving at 0m/s. The wagon would be moving away from him. It is 0m/s to a person on the ground. But yes, the ground frame of reference is the important one. The assumptions I have made is that engines make air speed. It moves the plane forward relative to the air. It can't "see" what the ground is doing. All it does is blow air backwards so the plane moves forward relative to the air. The second statement is that the wheels have minimal friction. Wheels are free to spin and the axles are very well oiled. Do you consider these correct?
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Post by Random on Oct 13, 2007 4:31:01 GMT -5
That's the trick. Can you do that? It will try to match the jets movement, but since the jet is going to move forward relative to the air, the treadmill spinning underneath will not be able to keep it stationary. I was saying that your swimming example isn't very good since frictional force from your feet drags you backwards and makes it hard to swim forward (assuming that one even needs to get a running start, which is false since we float). Wheels are in fact quite different from dragging your feet. There is a reason that cars have wheels and not sleds because sleds have a lot more friction. I'm not sure what you are getting at with the hot wheels car. If you move the car forward at a certain speed, you will always be able to do so. If you push the car forwards relative to the ground at 1m/s, then spin the conveyor belt backward at 1m/s, the car will move forwards relative to the ground at 1m/s, not stand still. K its 5 AM and you've managed to confuse me enough to get me to lose what I was getting at but I think I've more or less got it. Basically heres what I think you're saying. Once the engines turn on and all that crap the jet starts moving and then the wheels start turning and because they have minimal friction the wheels 'absorb' all of the negative velocity because the jet is pushing off of the air to move forward. Now, my thinking concerning the toy car as an example. You put it on a moving belt, and its wheels stay still. If you make it start moving 1 m/s relative to the conveyor belt the wheels will rotate to cover 1 m/s. If you make that car move 1 m/s relative to the GROUND (1 m/s - speed of the belt if you want to express it as relative to the belt), the wheels will rotate to cover 1 m/s - the speed of the conveyor belt (if the belt is moving at -1 m/s, the wheels will rotate to cover 2 m/s). Now, given the toy car example is true, for the jet to speed up relative to the air (which may as well be the ground as far as this goes) it ALSO inherently has to speed up relative to the belt, and as such the belt would merely need to spin faster in order to force a reduction in the jet's speed relative to both itself and the air/ground, and as I've said, in turn result in 0 lift. Heres kind of a timeline for thinking about it: 1. Jet has engines off, and is moving at 0 m/s relative to the air and ground due to the treadmill being off as well. 2. Jet turns on engines, begins moving at 1 m/s relative to the air/ground, and 1 m/s relative to the treadmill. 3. Treadmill turns on to compensate, moves at -1 m/s and as a result the jet begins moving at 0 m/s relative to the air/ground 4. Jet speeds up to 3 m/s relative to the treadmill, and 2 m/s relative to the air/ground 5. Treadmill speeds up to -3 m/s to compensate, as a result the jet begins moving at 0 m/s relative to the air/ground To respond to that, you would probably say that the friction would have no effect and the wheels would as you've said 'absorb' the additional speed from the treadmill. I'll be honest, I don't really remember most of my physics, but I'm absolutely positive that the wheels would act as I said they would in the toy car example, which illustrates that they don't work the way you're implying due to the fact that if the wheels DID absorb all of the movement, the car would just stay wherever you put it on the belt and never move. I think the wheels might absorb a fraction of the movement, but the treadmill would compensate for that based on the original example. I guess the way to summarize all that would be to say: there is no such thing as 0 friction. Can you describe what it was about? I'm curious to know what it is. To someone standing on the treadmill, the wagon is certainly not moving at 0m/s. The wagon would be moving away from him. It is 0m/s to a person on the ground. But yes, the ground frame of reference is the important one. The assumptions I have made is that engines make air speed. It moves the plane forward relative to the air. It can't "see" what the ground is doing. All it does is blow air backwards so the plane moves forward relative to the air. The second statement is that the wheels have minimal friction. Wheels are free to spin and the axles are very well oiled. Do you consider these correct? Basically I think the mistake you're making is that you're changing what something is being viewed by without changing the other variables as they would be seen by whichever observer. Essentially changing the point of reference or whatever you want to call it without changing all of the other variables involved such as the velocity of the person in the wagon example assuming the -1 m/s from the treadmill but then also having the motion of the wagon viewed by a person who does not perceive the -1 m/s and then comparing the two, which, if you can understand what I mean now, you probably agree is a massive flaw.
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Post by BlueDolphin on Oct 13, 2007 13:11:54 GMT -5
Now, my thinking concerning the toy car as an example. You put it on a moving belt, and its wheels stay still. If you make it start moving 1 m/s relative to the conveyor belt the wheels will rotate to cover 1 m/s. If you make that car move 1 m/s relative to the GROUND (1 m/s - speed of the belt if you want to express it as relative to the belt), the wheels will rotate to cover 1 m/s - the speed of the conveyor belt (if the belt is moving at -1 m/s, the wheels will rotate to cover 2 m/s). Now, given the toy car example is true, for the jet to speed up relative to the air (which may as well be the ground as far as this goes) it ALSO inherently has to speed up relative to the belt, and as such the belt would merely need to spin faster in order to force a reduction in the jet's speed relative to both itself and the air/ground, and as I've said, in turn result in 0 lift. Heres kind of a timeline for thinking about it: 1. Jet has engines off, and is moving at 0 m/s relative to the air and ground due to the treadmill being off as well. 2. Jet turns on engines, begins moving at 1 m/s relative to the air/ground, and 1 m/s relative to the treadmill. 3. Treadmill turns on to compensate, moves at -1 m/s and as a result the jet begins moving at 0 m/s relative to the air/ground 4. Jet speeds up to 3 m/s relative to the treadmill, and 2 m/s relative to the air/ground 5. Treadmill speeds up to -3 m/s to compensate, as a result the jet begins moving at 0 m/s relative to the air/ground It is number 3 and 5 that I disagree with. The airplane moves relative to the air, not the surface of the treadmill. The treadmill doesn't magically make everything in the volume above it move backward. When the treadmill moves -1m/s while the plane is moving at 1m/s, it would cause the wheels to spin 2m/s. The plane still moves forward relative to the air. You are correct that it must move faster relative to the belt. But this is trying to think through two reference frames. Airplanes don't care for their "ground" speed where "ground" is the belt surface. It does speeds up relative to the belt, but why would this matter? All that matters is that is speeds up relative to the air which is what the engines provide. In the hotwheels example, I was envisioning someone pulling it along the conveyor belt so that it moves relative to the ground. The conveyor belt cannot stop an external force from your hand since you are not attached to the belt. You are saying that if I push a car 1m/s from the ground (this is what you are measuring, not what a meter on the car would measure) and the conveyor belt moved backward at 1m/s that the car wouldn't move. This is a paradox since it was stated that the car is moving at 1m/s. It can't be standing still at the same time. I agree there is no such thing as 0 friction. But I can't envision wheels having so much friction that they drag you backwards. If the plane was on sleds, I would understand. Wheels on planes are not attached to the engines or anything. So the fact that the wheels spin backwards isn't sapping energy out of the engine. I agree with you that it is the movement from the ground that matters, not the treadmill. I included the person on the treadmill just for description. So I not trying to switch between the two. All speeds are measured from the ground. The 1m/s motion of the wagon and the -1m/s of the treadmill are measured from a person standing on the ground. Here is another thought experiment: Do you agree that all the treadmill can do is spin the wheels? If you do imagine a plane that is in flight at 10m/s just above the ground. Gremlins then come out and start spinning all the wheels simultaneously backward and pushing the wheels up to match the normal force of the plane at 10m/s. Will the plane now lose speed and crash?
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Post by Evilduck on Oct 15, 2007 1:08:04 GMT -5
Imagine that the plane's wheels have negligible friction at the axel (where wheel is connected to the plane).
Now, if the plane was held in place, the treadmill could go as fast as it wanted and the wheels could absorb all the speed, no force would go to whatever is holding the plane in place.
Thus, the jet engines could move the plane forward at the regular rate (relative to the ground) and the wheels would spin at that speed plus the treadmills speed. However, the plane would still go forward as if there was no treadmill.
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