>move with just the prop no matter what was holding them in>place. That's obviously not the way it is, so I doubted that>line of thinking.actually as a pilot stop and think - wheels are not absolutely necessary on the plane. Have you herad of planes on skis?>Does that mean that, in a>vacuum, a car would get better gas mileage the faster it went,>assuming a direct drive (non-geared) transmission?If car's engine could run in a vaccum (unfortunately normal engines needs air) then yes, you would get much better mileage because of lack of air resistance. At speeds say 60 mph and above air resistance is absolutely dominant force working against the moving car. Also your mileage would be pretty much independent of speed, not like today.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

Skis don't work on concrete though. Not as well as wheels at least. There's the whole friction thing again, right?What I meant about the car is, would it get better mileage at higher speeds than lower, since the only thing resisting it would be the friction between the wheels and the road? If friction does decrease as speed increases (and now I do find myself remembering this from junior high science), then that sounds like a logical assumption.

>friction does decrease as speed increases consider friction constant with speed. You may be confusing static friction to dynamic friction - lets' not go into this.> Skis don't work on concrete though. This is the problem with many people here - they confuse property of individual materials with fundamental laws of physics.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

>"Then there are others that treat it as a mere problem of>reference frames. This is probably the worst approach, as you>have to realize the dynamics of the system to get it, that>there is a net force in the forward direction.">>Talk about rubbish. The speed frame of reference in this>exercise is THE most critical point.It is THE most critical point in tricking the mind. To obtain a solution it is contraproductive to think of the reference frames. If you knew a little physics you would know that the dynamics is independent of reference frames. Thus the speed of the threadmill don't matter.I have enjoyed the debate too, having to explain my view helps keep the mind sharp. People seem to be inventing their own laws of physics though. But what do scientists know? They only split the atom and sent people into space.-

Plane Flys:It's all in the method of propulsion.Car propels car through the use of a drive shaft which imparts motion to it's wheels, The wheels will accelerate the car as long as traction is maintained. If the force imparted on the wheels exceeds the coefficient of friction between the wheels and the ground, the car stops accellerating. Which is why you can do awesome burnouts in your Boss Mustang GT 5.0.In an airplane, or jet powered car, the force of propulsion is not based on spinning the wheels but rather the force of the jet or prop, against the surrounding air. In this scenario, the plane and the treadmill will rapidly spin, but since the thrust is against the surrounding air, the plane will move forward (newtons law of motion)IF the propulsion method for the plane were simply through a "drive train" in which the force of the engine is transferred to the wheels directly, then the plane would never move, but since the thrust is applied to the surrounding atmosphere, then the wheels are irrelevant.The fact that your Boss Mustang GT 5.0 can burnout, skid, hydroplane, etc, actually proves the plane can fly. For a car or a person to move, it MUST push against the ground, For a plane to move, it must push against the surronding air.For the final thought on this question, take the treadmill and substitute a flowing river. A river moves 5 or 10 mph in a given direction. Does a seaplane have a shorter or longer takeoff run dependent on river flow? No. In this case the treadmill is like the river. Since a Seaplane does not have to rely on river flow to determine it's takeoff length, a wheel based plane on a treadmill would act the same.

Jeremy,Once the plane has overcome the small frictional force between its wheels and the treadmill itself, it will start to move forward. Whether the treadmill is powered or not (and I am still not sure what this is supposed to mean) is IRRELEVANT.Chris Low.

>>But what if you had say the Wright Flyer (with wheels) and a concrete >>runway moving backwards at 500 knots?It wouldn't go back at 500kts. For something as draggy and as light as the write flyer, it wouldn't reach 10kts (or so) backwards before the drag because equal to the wheel friction, but the wheels would be spinning at 490kts forwards.

Wathomas,Its alread been pointed out countless times in this thread but the river is a good analogy though.

I do know a "little" of Physics and for ONE last time, I'm not talking about Physics here. I understand your point and don't quarrel significantly with it, given the same set of assumptions.If you knew a little of language interpretation, you'd understand that I'm talking about different possible, but perfectly legitimate, interpretations based on wording. Different ways the situation could be set up BY DEFINITION. This is, after all, a mental exercise, not a real world event. Word interpretation has been at the core of these mind teasers for many, many years. Though misinformation continues to flow freely in this thread, I'm done with it. It was fun for a while, but the rarified atmosphere that a few of you think you live in grows tiresome.

>If you knew a little of language interpretation, you'd>understand that I'm talking about different possible, but>perfectly legitimate, interpretations based on wording.It is of course possible to imagine the problem in a way whereby the plane will stand continue to stand still. But the point is that even if such a situation is imaginable, it is PHYSICALLY impossible. So how can you disregard physical reality in one part of the problem, while trying to describe physical reality in another part of the problem (i.e. will it take off)?-

>It is of course possible to imagine the problem in a way>whereby the plane will stand continue to stand still. But the>point is that even if such a situation is imaginable, it is>PHYSICALLY impossible. Yes, that is right. Those of us who got it wrong took what we knew about a treadmill and applied it to the problem. This assumption was what threw us. I'm still getting a lot of people on this (they go through the same process I did). Initially many said "the problem was vaguely worded". I thought so too. However, when you look at it, we had all the information we needed.

Not to keep going on this, but someone else I sent this to brought up an interesting twist. Does "matching the forward speed of the plane" mean matching the speed of the airplane wheels spinning? If so, it is possible that the wheels would keep accelerating as would the treadmill. Both would accelerate until the airplane either took off, or the wheels fell apart from the speed. Of course as I now understand, the plane and the wheels would be spinning at different speeds, so which speed is the one that the treadmill matches? I am just thinking of a groundspeed gauge that reads the speed of the wheels. Although if this is acceptable, it still doesn't really change the answer much. Either the plane takes off still, or the wheels fall apart.

Will,Very good analogy with your river takeoff, however for some folks mental transition from water/floats to concrete/wheels is just way too hard to make.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

Wow, I just got this! It is possible. It was like a lightbulb went off in my head.

(can not be bothered to read the whole post, will take a week)i dont think it is possible.In order for a plane to take-off, there must be wind which helps lift the plane off.when on a tredmill, because you run on a opposing object (the moving path), you usually run at the same speed, and when both forces (speeds ni both directions) are equal, no wind would help the plane move up.now, unless the plane travels faster than the tredmill, then wind might eventually pick up, and the plane will take off. but then depending on whether the plane doesnt have any maximum speed, it will have to continue on getting faster and faster until wind will pick it up. Otherwise, if the plane reaches maximum speed, it wont take off.Think of it this way.Plane - 250 knotsTredmill - 240 knotsDifference in speed (i.e. total motion) = +10 knots Not very fast