Now this make a lot of sense, unfortunately it is because my tires provide the drive (thrust). In the case of a plane, the thrust of the engines do not transfer to the tires. The tires are not "drivers".

The tires are freewheeling. It matters not how fast the wheels are turning. Think about it for minute, for that period of time before a plane lands. The tires are going Zero RPM. Zero. An instant later, they are spining really fast to match ground speed because they are on the ground. If there was a speedometer on the tires you would see them go from 0 to 150 MPH (assuming landing speed was 150) instantly.

Sleep well, g. I have to work in the morning so won't be back to teach till tomorrow evening.

 

True

Not true. The wheels don't have to turn at all. Remember several months ago when a plane landed in California with the nose wheel lock at 90* to the body of the plane? It landed and the tires didn't turn at all, they just went up in a cloud of smoke and flames. Given enough thrust, the plane could move relative to the ground with the brakes locked and the tires skidding so tire speed does not HAVE to be related to ground speed. NORMALLY it is but it doesn't have to.

Not true. While this is true for a wheel driven vehicle, it is not true of a vehicle powered by the thrust of a jet engine that pushed against the air rather than against the ground.

You're on to something here but I'm not sure that you realize it. If rolling resistance is equal to thrust (ie. the brakes are applied and the tires have adequate traction), you could theoretically keep the plane from taking off but since no brakes are applied in the theroetical case that we are discussing (the plane is trying to take off), the only resistance that could work against that thrust is the wheel bearings and the friction of the tire on the pavement/conveyor and in no case will that be adequate to counteract the thrust.

True for a wheel driven vehicle BUT, take the same truck, throw it in neutral, and strap a solid fuel rocket in the bed. Now will the conveyor keep the truck from moving? NO. The conveyor may be doing 50 or 90 MPH but the rocket is going to push that truck down the conveyor belt bacause it's pushing against air rather than against the conveyor.

 

Exactly right 12 valve. Here's it is another way: The conveyor can not affect the forward momentum of the plane, like it was said before, because the they are not drive wheels on the plane. They will be spinning just as fast as the conveyor (opposite direction), but the plane will speed up because it's not pushing against the conveyor, it's pushing against the outside air.

Eventually the plane will reach 150 mph air speed. The wheels will be traveling backward at 300 mph. Plug in any numbers you want, but it still works. The plane will get air speed, and the wheels will spin faster. The plane WILL take off.

Somebody get a radio controlled air plane and a treadmill and solve this once and for all!!!

 

On any conveyor belt in the world, the plane would take off. However, there is not a conveyor belt in the world that can match the plane's tires' exact speed.

Even if you had a truck in neutral on a conveyor belt that was traveling at 50mph and strapped a hook to it and pulled it forward at 10mph relative to the ground, the wheels would still be turning at 60mph, whether they are driven or not.

The brakes may not be applied, but rolling resistance is always present. It would take a HUGE amount of speed to get enough resistance to equal thrust. The tires probably would start sliding before that, but the plane will still not gain enough momentum to take off.

 

Wow! This is some serious discussion. I would like to ask this to the people that don't think that the plane will take off:

Since the earth is rotating at about 1000 MPH is it important to put the plane perpendicular to the direction of rotation? I mean hey if the ground is moving that fast than it would take 1000 extra MPH facing west than it would facing east right??????? Think again about the relativity thought experiments.

Tengu

 

Okay...this was humorous at first. Now it's getting old.

 

I dont think this is old at all. I enjoy thinking about it and discussing it with others, who you feel are right and who I feel are wrong.

The speed of the wheels and conveyor belt DO have a great deal to do with this. The plane cannot move forward without the wheels traveling faster than the conveyor belt.

If you have a stationary toy car (relative to the ground) on a moving conveyor belt, the belt will spin the wheels. The outside of the tire will spin at the same speed as the top of the conveyor belt. Now accelerate the vehicle by some outside force (your finger in the toy car analogy). The outside of the wheels MUST move at a greater speed than the conveyor belt is moving for the vehicle to move in the direction of the force applied, unless the tires are sliding.

Also, in the toy car analogy, the car will obviously not stay in one spot on the conveyor belt without you holding it. Why? Rolling resistance. I'm not saying rolling resistance will overcome thrust at some normal, possible speed, but eventually it will.

 

wow...thats all I have to say about this thread...wow

by the way....

12valve@heart knows his stuff (for those of you who still think the plane will not take off)

 

ok, last thing i will ever say on the thread, think of a plane in the air, yes it is flying at say 100 mph, now the plane hits a 100 mph tail wind. what happens? the plane ceases forward progress and descends because of no lift. es todo, no mas from me

 

Come on, don't give up.

I had a guy with a Masters in physics today tell me that the plane would not take off, so you're in good company.

Basically, if the conveyer belt is moving exactly the same speed as the airplane's wheels, then the plane would keep the wheels spinning until the rolling friction equals the thrust, or something like that...

But in the pragmatic world, we all know what would happen...


This just in:
I posted the brain teaser on the politics forum that I belong to; everybody over there's a total gadfly or devil's advocate, so it might be fun to watch them go at it from a distance...

http://www.uspoliticsonline.com/foru...ad.php?t=23843

 

Alleluia! Finally someone to back up what I've been saying all along!

I understand what you guys have been saying, and I know it is impossible in this world, but assuming the conveyor belt is always traveling at exactly the same speed as the tires, the plane won't take off.

 

Derek, 100 mph tail wind hits a plane going 100 mph (airspeed) the plane will now be doing 200 mph. This happens to me all the time, but not to those extremes.

Also think of this.... flying along at 100 mph with a 100 mph headwind. Your airspeed is still 100 and your ground speed is 0 mph, but you are still flying. Slow your airspeed to 80 mph and you are going backwards 20 MPH. I have done this several times. Flying backwards is a goal of many light aircraft pilots.

Don't go away.... stick around. We'll learn something togeather, and have fun doing it.

 

I can't believe I just joined a forum to argue the physics of an airplane taking off from a conveyor belt. I must lead an exciting life.

Anyway to the question at hand:

The part of the question I believe to be tripping people up is "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation."

We all know that this is impossible but for the sake of the question assume it is true. Also assume there is no slip between the wheels and conveyor and the conveyor always travels in the opposite directon of the wheels. The speed of the aircraft relative to the ground is equal to the speed of the wheels minus the speed of the conveyor. If the plane is to have a forward velocity then there must be a difference between the conveyor speed and the wheel speed. If the plane is to take off the difference must be equal to or greater than the take off speed. Given the problem statement "the conveyor belt is designed to exactly match the speed of the wheels" the difference in the two must be zero therefore there will be no ground speed. If there is no ground speed there is no lift. The plane will not take off.


As for the plane on skis argument, if you make the same assumption of zero slip they wouldn't take off either. The fact that the skis slide on the surface allows the plane to accelerate and eventually take off.

 

HEY!!!! First time posters are not allowed to post logical and seemingly well thought out reponses!!!

So, how much did Gman pay you to come on here and say that??

Besides, what the original poster forgot to mention is that the plane ALSO has helicopter blades on top.

 

He has owed me for a long time! Way back when he was moving out of his old duplex (way back in the summer) I helped him scrub walls and floors and such. Of course that may have been because he is my brother...

Well if you won't admit that, will you AT LEAST admit that you don't actually have a concrete bedliner?