hyman shoulder wrote:
Why is this still such a controversy? Everyone with a brain has concluded that it's the engines that cause a plane to take off. The wheels will basically be spinning freely, and it won't matter what speed they're going. The plane takes off.
Cars have engines, but they don't fly, no matter how fast you drive, and whether they are on a treadmill or not. Engines create thrust, and wings turn thrust into lift, but only if air is moving over them.
Poster A: Planes don't take off without air moving over the wings.
Poster B: The plane takes off because air is flowing over the wings.
Poster A: No, the plane can't take off without air flowing over the wings.
Poster B: Air IS flowing over the wings!
Poster A: You are a moron if you think the plane can take off without air flowing over the wings.
etc. etc. etc. There is some masterful trolling going on here.
i love this riddle. it took me about an hour to spot the red herring in the question. It takes getting over a couple preconcieved notions but it most certainly takes off.
I either misunderstand the setup, or if it does indeed take off, I need to quit physics forever because I don't get it.
Physics Grad wrote:
I either misunderstand the setup, or if it does indeed take off, I need to quit physics forever because I don't get it.
Indeed.
hmmm, it was kind of hard for me for a while. the best thing i can say is to imagine a 10 ft long treadmill, and there is a rope tied to the wall infront of it, and you are wearing rollerblades. now step onto the end of the treadmill while holding the rope, and turn the treadmill on to whatever speed youd like.(lets say 8 mph it doesn't matter) now starting at the end of the treadmill you pull yourself forward at 10 feet per minute. this simulates airspeed, your getting 10 feet per minute airspeed, even though the treadmill is going 8 miles per hour backward(much much faster than your the rate you move forward at, the converyor belt in the riddle only matches the airspeed at which you travel forward.) So after one minute you arrive at the end of the treadmill despite the treadmill going backwards at a much faster rate than the rate you move forwards at. now imagine a treadmill the length of a runway. This example is designed to show the nature of the forces at hand. the rope from the wall simulates the nature of the force the engine of the plane exerts on the air. and the roller blades simulate how the planes wheels just spin freely. in the example you did get airspeed(the airspeed of going from one end of the treadmill to theother) the red herring in the question is saying that the conveyor belt in question matches the speed of the plane, because it could really be any speed and it wouldn't matter(like the roller blade case). it doesn't negate the planes forward movement along the runway sized conveyor belt. remember running on the treadmill or putting a car on a treadmill is different, because both get their forces as a result of the direct contact with the ground, whereas a plane doesn't. so the treadmill would negate them. ok i think thats all for me i think this debate will go on forever.
It is simple once you understand. The treadmill doesn't keep the plane still - yes the treadmill must be the same length of a runway to provide enough space for the plane to take off - but the backward motion of the treadmill doesn't overcome the engines of the plane because the treadmill is working against free spinning wheels not the engine.
For a person the "thrust" is being produced by the legs contacting the surface of the treadmill - this is not true with a plane. Another way to think of it is that a wind tunnel is a "treadmill" for engines.
I, too, thought that the plane would not take off, but now I am not so sure. We should be careful in physics to not let our pre-concieved notions sway our belief in what is going to happen. How many times, when we were first learning physics, did this happen? Let reason be our guide. Physics, many times, goes agains common sense. This is one of those times. The plane will take off.
Neb wrote:
It is simple once you understand.
Duh ;)
The treadmill doesn't keep the plane still - yes the treadmill must be the same length of a runway to provide enough space for the plane to take off - but the backward motion of the treadmill doesn't overcome the engines of the plane because the treadmill is working against free spinning wheels not the engine.
For a person the "thrust" is being produced by the legs contacting the surface of the treadmill - this is not true with a plane.
Ah, see? If this is the setup, then that is what I misunderstood. I haven't followed all of the threads about it so I imagine I missed this explanation. If this is the way it is, then of course it can take off!
The way I read it was like that of a person on a treadmill where you have no forward motion with respect to the floor.
Either way, thanks for that explanation! And if thats the way it is, I can stay in physics yay!
Thanks for yours too Neb
purveyor of spirits wrote:
The people who say it will take off say this:
They argue that a plane is different. The airplane doesn't move based on the tires pushing off the road. It's the engines that push it. The engines are only "connected" to the air. If you remove the runway ***AND SUSPEND A PLANE ON A STRING,*** it will still thrust forward.
Where the hell is this damn magical "string" in the actual experiment? What happens in real life, when there is no "string" holding the plane up? That's right, GRAVITY keeps it down. That's why a plane needs wheels - to be able to impart the push of the exhaust on the engine housing to the runway until the plane has accelerated to a point where the airspeed over the wings creates enough lift to overcome gravity (which can be measured in Newtons as mass of the plane x gravitation accel. ~9.8m/s/s). until that force is overcome, that is, until the airplane is actually flying, the car comparison doesn't work.
i you-tubed some roller-skates on treadmill videos a while ago, and while i don't feel like doing it again, you could easily see that the person on the roller skates had to hold on to the frame of the treadmill to stay in place - if he let go he would start to be moved backward by the belt due to the friction between the wheels and the belt, and this is also taking into account the fact that the wheels were already spinning when the roller-blader lets go; if he were simply standing on the treadmill in the skates when he turned it on, it follows that he would be accelerated backward even more quickly. now, if the wheels of a roller skate with the weight of a kid on them are not free-spinning due to friction, what on earth makes you people think that the wheels of airplane would be?
If the treadmill matches the wheel speed, the plane will not take off. That's the truth.
I think the key element I missed initially was that the treadmill is the length of a runway and the plane is allowed to move forward (and therefore attain airspeed)
Re: Party time and wheel
Agreed!
keystone light knight wrote:
purveyor of spirits wrote:The people who say it will take off say this:
They argue that a plane is different. The airplane doesn't move based on the tires pushing off the road. It's the engines that push it. The engines are only "connected" to the air. If you remove the runway ***AND SUSPEND A PLANE ON A STRING,*** it will still thrust forward.
you could easily see that the person on the roller skates had to hold on to the frame of the treadmill to stay in place -
exactly. and he barely has to exert any force to hold on, much less pull himself forward(getting airspeed as he goes along the treadmill) ok now i'm really done
Yeah, I'm now on the "it will take off" side. I thought the thought experiment basically involved ensuring that the plane would not move, i.e. the infinitely accelerating treadmill. If that were the case, then no it wouldn't take off. Otherwise, a normal treadmill will not provide sufficient force to prevent forward motion of the plane.
Even with the infinitely accelerating treadmill, I still like my argument about it moving the air above fast enough to give the plane lift
So let's say a plane needs to reach 150 MPH to take off. You have crazy long treadmill running at 150 MPH. The plane reaches 300 MPH (measured by wheel spin), thus a forward speed of 150. It lifts off, but the moment its wheels leave the treadmill does it suddenly get 300 MPH worth of thrust even though its only moving forward at 150? That would be worth watching.
wheel wrote:
If the treadmill matches the wheel speed, the plane will not take off. That's the truth.
Listen, this guy has it right. Most of you "plane will take off" folks believe the engine thrust will override wheel-treadmill friction and the plane will move forward (accelerating) despite the wheels moving, however fast. BUT, if the treadmill can keep up and match wheel speed, then wind lift = 0. No flight.
It all depends on the nature of the treadmill.
keystone light knight wrote:
That's why a plane needs wheels - to be able to impart the push of the exhaust on the engine housing to the runway
Anyone know if this is true? The basic premise of the "it takes off crowd" is that the wheels are not powered.
I think its an action-reaction pair of forces. The tires on the runway, and the runway on the tires, etc.