Page 2 of 5 FirstFirst 12345 LastLast
Results 21 to 40 of 97
  1. #21  
    Quote Originally Posted by aprasad View Post
    the treadmill matches the speed of the airplane: What is the reference frame in which the speed is being measured?

    The most common interpretation: the reference frame is attached to the ground => Plane will take off
    In that case, yes. The treadmill will then roll backwards at half the speed of the airplane relative to the treadmill and thus be the same as the speed of the airplane relative to the ground (in the opposite direction):

    velocity of the plane relative to the ground = -(velocity of the treadmill)
    velocity of the plane relative to the treadmill surface = 2 x velocity of the plane relative to ground.

    If the plane can drive at twice the normal takeoff speed, it can take off.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  2. #22  
    Quote Originally Posted by clulup View Post
    That is true, but for gaining speed on the treadmill it makes no difference that the plane is powered by jet engines and not via the wheels. Acceleration works just as well with wheels powered like those of a car - at least until takeoff, then the wheels would be no good anymore and the plane would come down again.
    That's your fundamental mistake. The fact that the wheels aren't powered DOES make a difference. A huge difference.

    In a car, it would work the way you describe, because the treadmill offsets the power being applied to the wheels. With the wheels freely spinning (on the plane), the treadmill offsets nothing.

    Why do you think the physics of motion suddenly changes, dramatically, when the plane's tires are 1/1 millionth of an inch off the ground? The tires are a red herring--they have nothing to do with the physics of the plane's motion.
    Bob Meyer
    I'm out of my mind. But feel free to leave a message.
  3. #23  
    Quote Originally Posted by meyerweb View Post
    That's your fundamental mistake. The fact that the wheels aren't powered DOES make a difference. A huge difference.

    In a car, it would work the way you describe, because the treadmill offsets the power being applied to the wheels. With the wheels freely spinning (on the plane), the treadmill offsets nothing.
    We have been discussing two different behaviours of the treadmill:

    1. Treadmill has the same velocity as the **airplane relative to the TREADMILL SURFACE** (but in opposite direction)
    Example: plane rolls at 100 mph relative to the treadmill, the treadmill rolls at 100 mph into the opposite direction.
    Result: the plane stands still relative to the ground/to the air, no takeoff. This is basically the situation encountered in the fitness studio: treadmill moves, guy runs, but stays in the same place relative to the fitness studio. It makes not difference whether a plane or a car or a person is involved.

    2. Treadmill has the same velocity as the **airplane relative to the GROUND** (but in opposite direction)
    Example: plane relative to the ground at 50 mph, treadmill surface realtive to ground -50 mph, airplane relative to treadmill surface 100 mph: 100 mph -50 mph = 50 mph.
    In this case the airplane can lift off if it can roll with twice the takeoff speed.

    However, also the second case is independent of how the plane (or car) is powered. Also a car could reach takeoff speed of an airplane on the treadmill if the engine and the tires are strong enough, it does not need a jet engine or a propeller to do that.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  4. #24  
    Quote Originally Posted by meyerweb View Post
    Why do you think the physics of motion suddenly changes, dramatically, when the plane's tires are 1/1 millionth of an inch off the ground?
    Of course it makes a tremendous difference whether the wheels touch the ground or not. Lift your car so that the wheels are 1 millionth of an inch off ground. I'm sure you will notice the difference.

    Neverthelass, as long as the wheels touch the ground and there is friction, a car can drive on a treadmill without any problem. Of course the car would come down again immediately after takeoff (assuming it has wings) because it slows down again, but still...
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  5. #25  
    Quote Originally Posted by clulup View Post
    We have been discussing two different behaviours of the treadmill:

    1. Treadmill has the same velocity as the **airplane relative to the TREADMILL SURFACE** (but in opposite direction)
    Example: plane rolls at 100 mph relative to the treadmill, the treadmill rolls at 100 mph into the opposite direction.
    Result: the plane stands still relative to the ground/to the air, no takeoff. This is basically the situation encountered in the fitness studio: treadmill moves, guy runs, but stays in the same place relative to the fitness studio. It makes not difference whether a plane or a car or a person is involved.

    2. Treadmill has the same velocity as the **airplane relative to the GROUND** (but in opposite direction)
    Example: plane relative to the ground at 50 mph, treadmill surface realtive to ground -50 mph, airplane relative to treadmill surface 100 mph: 100 mph -50 mph = 50 mph.
    In this case the airplane can lift off if it can roll with twice the takeoff speed.

    However, also the second case is independent of how the plane (or car) is powered. Also a car could reach takeoff speed of an airplane on the treadmill if the engine and the tires are strong enough, it does not need a jet engine or a propeller to do that.
    1. Will not happen because the thrust from the engines will move the plane forward relative to the ground.. no matter what the treadmill top surface does. So, picture the plane's turbine engines powering up and the plane rumbling down this loooong treadmill. This will happen.
    What the motors on the treadmill do: stay still, move the top surface rearward to match the plane speed, move in a random way, whatever.. will affect the rpm of the tires, which are free-rolling.

    If it is easier, replace the tires by a frictionless ski-sliders (line snow-planes have) sliding on the treadmill. Now do you have any doubts whether the plane will take off?

    The plane will lift off exactly like it always does, with the same ground-speed. Regardless of what the treadmill operator does with his controls
    Last edited by aprasad; 12/08/2006 at 08:23 AM.
    --
    Aloke
    Cingular GSM
    Software:Treo650-1.17-CNG
    Firmware:01.51 Hardware:A
  6. #26  
    Quote Originally Posted by clulup View Post
    In that case, yes. The treadmill will then roll backwards at half the speed of the airplane relative to the treadmill and thus be the same as the speed of the airplane relative to the ground (in the opposite direction):

    velocity of the plane relative to the ground = -(velocity of the treadmill)
    velocity of the plane relative to the treadmill surface = 2 x velocity of the plane relative to ground.

    If the plane can drive at twice the normal takeoff speed, it can take off.
    This illustrates perfectly why there was such debate at airliners.net over this problem. The person who posted it there either worded it wrong or found it that way; someone else found the original problem (conveyor matches airplane's speed--not wheel speed--relative to Earth, in the opposite direction), and there wasn't much debate on that at all. Assume frictionless wheels and tires rated for infinite speed, and it doesn't matter what the belt does; it can run at any speed in either direction, and the plane will just get up and go as normal. Add friction, and that becomes the only factor.

    (Another question they debated at length is whether or not a huge airplane filled with birds would weigh less if the birds were flying around inside or sitting...)
    "Yeah, he can talk. It's gettin' him to shut up that's the trick!"
    -Shrek
  7. #27  
    Quote Originally Posted by aprasad View Post
    1. Will not happen because the thrust from the engines will move the plane forward relative to the ground.. no matter what the treadmill top surface does. So, picture the plane's turbine engines powering up and the plane rumbling down this loooong treadmill. This will happen.
    What the motors on the treadmill do: stay still, move the top surface rearward to match the plane speed, move in a random way, whatever.. will affect the rpm of the tires, which are free-rolling.

    If it is easier, replace the tires by a frictionless ski-sliders (line snow-planes have) sliding on the treadmill. Now do you have any doubts whether the plane will take off?
    "replace the tires by a frictionless ski-sliders"... Tires of planes and cars and ski-sliders are by no means frictionless. If they were, you would hardly use any gas when driving steadily on a road. Nobody said the tires in the original problem are friction-less.

    Besides, do you doubt a man running on a treadmill rolling at the same speed in the opposite direction stands still relative to the fitness studio? Why would the plane be different? In case you forgot: a plane running on the runway does NOT fly or float above the runway. It is tons of metal resting on wheels with friction. In case (1) above, it stands still relative to the air like the runner on a fitness studio treadmill, so there is zero lift and no takeoff.

    In case (2) above, it can move relative to the air and may take off if it can roll fast enough.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  8. #28  
    Because the the force of propulsion in cars and the man (running) comes from the friction at the road/treadmill surface. The plane does not. That's why ski-planes can take off on ice.

    A plane with it's engines gunning (the way it normally does while taking off) will NEVER stand still relative to the ground, regardless of any super treadmill doing whatever the treadmill operator desires.

    What you describe is true for cars, not airplanes. Do you agree that the two cases (cars vs airplanes) are different from each other when it comes to operating on treadmills.
    Last edited by aprasad; 12/08/2006 at 09:22 AM.
    --
    Aloke
    Cingular GSM
    Software:Treo650-1.17-CNG
    Firmware:01.51 Hardware:A
  9. #29  
    Basically a plane blows itself forward by ejecting air.
    So when the plane takes of at 10% trottle it would do speed X, the treadmill would respond by going -X but since there is nothing (appart from friction of the wheels) stopping the plane going forward the plane would go at X MPH.
    If the pilot puts the trottle to 100% the plane would reach an airspeed of 10X which should be enough for liftoff (assuming it doesnt have a gailforce tailwind)
    <IMG WIDTH="200" HEIGHT="50" SRC=http://www.visorcentral.com/images/visorcentral.gif> (ex)VisorCentral Discussion Moderator
    Do files get embarrassed when they get unzipped?
  10. #30  
    Quote Originally Posted by ToolkiT View Post
    Basically a plane blows itself forward by ejecting air.
    So when the plane takes of at 10% trottle it would do speed X, the treadmill would respond by going -X but since there is nothing (appart from friction of the wheels) stopping the plane going forward the plane would go at X MPH.
    If the pilot puts the trottle to 100% the plane would reach an airspeed of 10X which should be enough for liftoff (assuming it doesnt have a gailforce tailwind)
    You forget that the treadmill reacts to your speed (scenario 1 above, treadmill goes at the speed the plane would have on normal ground under normal conditions, but in opposite direction. I think also ToolkiT and aprasad agree that at least a car does not move relative to the ground under those conditions (man on a fitness center treadmill scenario).

    Let's look at the airplane situation again: think of a really big, wide, long treadmill crossing the aequator. An airplane is standig there (aequator), getting ready for takeoff. The treadmill starts rolling south, until it reaches 100 mph. Does the airplane stay at the aequator, or does it roll south with the hughe treadmill? I guess we agree, it rolls south. Once the treadmill reaches 100 mph, the pilots check the windspeed. For them, it looks like there is wind coming at 100 mph from the south (from behind).

    They start the engines and put the gas in a position where the plane reaches 100 mph on a normal surface. Now, they roll at 100 mph on the treadmill. Relative wind is zero now, right? They push the gas lever to a level which results in a speed of 200 mph on the surface. Relative wind is now 100 mph from the north, they approach the aquator again, because they are faster than the treadmill now.

    But when they reach the aquator, the treadmill increases speed to 300 mph. The treadmill carries them south again, because their gas is set to rolling at only 200 mph. They feel the wind coming from behind again, at 100 mph. Damn, the pilot says to the co-pilot, more gas! They put the gas on the 400 mph positon, and since that is faster than the huge treadmill they are standing on, they approach the aquator again - but then the floor starts going south at 500 mph.

    The engines are already running at a force which allows rolling at 400 mph, but the wind is again coming form the back at 100 mph, so zero lift, the plane is still fully grounded. The pilot get nervous... "Push the zero friction putton, he yells". The co-pilot checks the manual. "Sir, there is no zero friction button!?"

    You get the drift?
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  11. PSB22's Avatar
    Posts
    192 Posts
    Global Posts
    203 Global Posts
    #31  
    I say the plane nose-dives forwards at the runway.

    Just like the human treadmill/fan/roller-blade analogies, having a treadmill spin the plane wheels backwards no matter how fast they're spinning forwards is akin to locking the wheels. i.e. no matter what happens, the wheels aren't going anywhere. Now, think about the force still being generated by the fan on your head / engine on the plane... if it is high enough off the ground, it will simply force you to keel over, and in the plane's case, it will nose dive.
  12. #32  
    Quote Originally Posted by clulup View Post
    You forget that the treadmill reacts to your speed (scenario 1 above, treadmill goes at the speed the plane would have on normal ground under normal conditions, but in opposite direction. I think also ToolkiT and aprasad agree that at least a car does not move relative to the ground under those conditions (man on a fitness center treadmill scenario).
    IT is irrelevant what the threadmill does since it is not connected to the airspeed of the plane.
    If the plane is on a normal runway the runway would 'react' by have a speed of 0 and the plane would push itself through the air and being slowed down by air friction (cD) and the friction of the wheels.
    Since the friction of the wheels wont vary much regardless of the speed and the Cd is a constant the rest is an irrelevant red herring..
    <IMG WIDTH="200" HEIGHT="50" SRC=http://www.visorcentral.com/images/visorcentral.gif> (ex)VisorCentral Discussion Moderator
    Do files get embarrassed when they get unzipped?
  13.    #33  
    Quote Originally Posted by clulup View Post
    That's not true. The wheels of a plane may spin freely once the plane is in the air, but before that, they are subject to friction exactly like the tires of a car.
    Well, not exactly like the tires of a car, if the car is in gear. Perhaps like a car in neutral. The force of friction will be due to axle grease, not rubber against the ground. To get a sense of the strength of that friction from axle grease, imagine a car in neutral sitting on a moving treadmill. You'd be able to stand at one end and push the car - not only stopping the car, but moving it in the opposite direction of the moving treadmill. That should tell you that the horizontal force exerted by the moving treadmill on the car in neutral is smaller than the force you were able to exert on the car - a pretty trivial amount relative to the force it takes to accelerate a car to say, 275 km/h.

    Of course, since a plane is bigger than a car, the force of friction will be bigger - roughly in proportion to their respective weights - but it'll still be a trivial amount relative to the force required to make the plane accelerate to 275 km/h. For a vivid illustration of the force exerted by a plane's engines against the air, check out this video:




    A knowledge of physics is a good thing, but without knowledge of the problem at hand, it can lead astray... As it said in the first post, "the treadmill has a clever design and always matches the speed of the plane, but runs in the opposite direction."

    If that means the treadmill will always run at the speed the plane would have on normal ground, but into the opposite direction, the plane will not move relative to the air and not take off. I think this is clear and not disputed.
    Nope. Very disputed!

    Note that the treadmill matches the speed of the airplane, NOT the force of its engines. It won't even be close. It'd be like trying to stop a race car with the friction of sandpaper moving in the opposite direction at the same speed. The reason people are ignoring friction is because it's a trivial factor.
    Last edited by samkim; 12/08/2006 at 07:57 PM.
  14. #34  
    Quote Originally Posted by samkim View Post
    Well, not exactly like the tires of a car, if the car is in gear. Perhaps like a car in neutral. The force of friction will be due to axle grease, not rubber against the ground. To get a sense of the strength of that friction from axle grease, imagine a car in neutral sitting on a moving treadmill. You'd be able to stand at one end and push the car - not only stopping the car, but moving it in the opposite direction of the moving treadmill. That should tell you that the horizontal force exerted by the moving treadmill on the car in neutral is smaller than the force you were able to exert on the car - a pretty trivial amount relative to the force it takes to accelerate a car to say, 275 km/h.
    This shows where your reasoning goes astray. Rolling friction is a "trivial amount relative to the force it takes to accelerate a car"? If that was true, a car rolling at a constant speed of, say, 50 mph, would not use any power to do that. That would be cool, but it is not how real life physics work.

    Also, your example of standing on the ground at the end of a treadmill with a car on it does not work as you say. Holding a car at the end of a treadmill rolling at x mph towards you takes the exact same amount of force as pushing the car at x mph on the ground. Ask an expert, he/she will tell you the same.
    “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
  15.    #35  
    Quote Originally Posted by clulup View Post
    This shows where your reasoning goes astray. Rolling friction is a "trivial amount relative to the force it takes to accelerate a car"? If that was true, a car rolling at a constant speed of, say, 50 mph, would not use any power to do that. That would be cool, but it is not how real life physics work.
    Because it's true, a car is able to easily overcome the force of friction to accelerate. Because it's true, you're able to overcome the force of friction to accelerate the car by pushing it.

    The fact that a car rolling at 50 mph with its engine shut off would slow down proves that the force of friction is greater than zero. But it doesn't change the fact that it's a trivial force relative to the power of the engine.


    Also, your example of standing on the ground at the end of a treadmill with a car on it does not work as you say. Holding a car at the end of a treadmill rolling at x mph towards you takes the exact same amount of force as pushing the car at x mph on the ground. Ask an expert, he/she will tell you the same.
    You're confusing speed with acceleration. Force is proportional to acceleration, not speed. If you can hold a car on the treadmill at 2 mph, you can hold it at 50 mph. You'd be fighting roughly the same amount of friction. (Heat might affect the friction.) The bottom line is that you're stronger than axle grease friction.

    The more interesting question (which doesn't change the relative strengths of friction, you, and a plane engine) is whether you'd be able to offset the force if the treadmill were to accelerate really quickly from 2 mph to 50 mph. Well, that would depend on the level of acceleration. But regardless, the wheels would begin to spin before you're overcome, since you're stronger than axle grease friction.
  16.    #36  
    Quote Originally Posted by Tom LaPrise View Post
    (Another question they debated at length is whether or not a huge airplane filled with birds would weigh less if the birds were flying around inside or sitting...)
    Cool question.

    If the plane were sealed, air-tight, then the air and birds would be like fish in a fishbowl. It doesn't matter if they're "floating" in the air or sitting on the bottom, at least on average. I suppose the internal air turbulence from the flapping wings could make the perceived weight volatile.

    If the plane weren't air-tight, then a flying bird would add less weight to the plane than a sitting bird. The flying bird adds some weight greater than zero because it's pressing down on the air, which presses down on the plane. We all carry the atmosphere above us.
  17. #37  
    Here's another question:

    If you were standing on a sensitive weighing scale at the edge of a cliff, peeing. Will you weigh more, or less, or the same? What if the pee has not hit the bottom of the cliff? Assume that the pee is in laminar flow and ignore the effect od air friction and turbulence.
    --
    Aloke
    Cingular GSM
    Software:Treo650-1.17-CNG
    Firmware:01.51 Hardware:A
  18.    #38  
    Quote Originally Posted by aprasad View Post
    Here's another question:

    If you were standing on a sensitive weighing scale at the edge of a cliff, peeing. Will you weigh more, or less, or the same? What if the pee has not hit the bottom of the cliff? Assume that the pee is in laminar flow and ignore the effect od air friction and turbulence.
    I know very little about fluid dynamics, but I'm pretty sure you'd weigh less. For two reasons. First, the pee is accelerating downward, and so is no longer sitting in your body, helping to press down on the scale. Whether or not the stream is in a free-fall, that would be the case.

    Second, the propulsive effect of the peeing would reduce your weight on the scale. The act of peeing isn't like opening up a valve and letting gravity take effect. The body forces the liquid out, and so like a mini-jetpack, peeing would put an upward force on your body.
  19. #39  
    What if you were peeing into a bucket that you were holding?
    --
    Aloke
    Cingular GSM
    Software:Treo650-1.17-CNG
    Firmware:01.51 Hardware:A
  20. #40  
    Quote Originally Posted by aprasad View Post
    What if you were peeing into a bucket that you were holding?
    ...while standing on, and in motion on a treadmill that just had a jet plane take off from?




    click photo to see this fabulous video, if you haven't already seen it...
    `
    `
    `
Page 2 of 5 FirstFirst 12345 LastLast

Posting Permissions