A harmless little riddle

R. Kelly,

Its time to come out of the closet. Never be ashamed of who you are. (Unless you're a bad person)

Tank, I do believe that you agree that the plane will fly, meaning there may still be hope for you.

But then you turn around an encourage Mr.I rape minors to come out of the closet. Please don't encourage him to come out, I was hoping the closet was airtight.

I don't always keep up on all the posts, so I'll take your advice and be more careful with my encouragements.

the riddler wrote:

An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?

Clarification: When the plane "starts its engines" there are two possible means of propulsion, wheel-driven and jet propulsion. Planes use both when on the ground before take-off. As long as the plane is operating strictly on wheel-driven propulsion and the treadmill keeps up with it, it will never go anywhere and will not take off. When it switches over to 100% jet propulsion, it will move forward from the treadmill irrespective of the speed of the treadmill and take off in a normal manner.

In the former case The plane's speed is considered relative to the belt of the treadmill, from which it derives its forward momentum. In the latter case the wheels' relationship to the treadmill belt is irrelevant and coincidental, as its momentum and forward thrust is derived from pressure against the air.

The misunderstandings in this post are due to talking about two different things as though they were the same. That, and some ignoramuses who just don't understand enough to even be discussing this.

tank wrote:

You saw it clearly fairly early.

Thanks, but I didn't. I fell for the "trick".

When I read the explanation I felt like a donkey.

I trained as an A/C mechanic, but have been out of it for 13 years. Still, it's no excuse.

Too many people in here are heading for the wrong jobs/majors me thinks.

hahahah that pic is AMAZING

I want to se his face when he finds out the truth

I'm real sorry, this is boggling my mind. According to the it'll fly theroy, shown by the motorcycle thing (which was awesome), a plane sitting on a giant tread going 150 mph backwards WITHOUT ITS ENGINES ON will stay in one place. This defies logic, it should roll backwards with the treadmill. I think this is indisputable. However, the "it'll fly" argument says that, once the engines are turned on, even AT ALL, it will begin to move forward equal to the thrust of the engine... and I'm having a hard time wrapping my brain around that.

the duder wrote:

I'm real sorry, this is boggling my mind. According to the it'll fly theroy, shown by the motorcycle thing (which was awesome), a plane sitting on a giant tread going 150 mph backwards WITHOUT ITS ENGINES ON will stay in one place. This defies logic, it should roll backwards with the treadmill. I think this is indisputable. However, the "it'll fly" argument says that, once the engines are turned on, even AT ALL, it will begin to move forward equal to the thrust of the engine... and I'm having a hard time wrapping my brain around that.

There is a small component of friction between the wheels and the treadmill ensuring that the plane will move backwards if it supplies no forward thrust. However the jet propulsion of the plane will easily overcome this friction ensuring that with enough forward thrust, the plane will move forward and takeoff. Imagine the dude with roller skates on the treadmill. If he stands still on the treadmill and the treadmill is turned on, he will move backwards with the treadmill. This backwards momentum will easily be overcome by the power of the motorcycle. And its important to mention that the motorcycle will not need to move forward at an equivalet speed to the treadmills rearward speed simply to overcome the frictional backwards momentum. I'm no physics major so somebody with a better fundamental understanding is likely to crush this explanation but thats how I see it.

Ignore those examples.

Imagine it this way-

A plane is on a runway(a normal one), and the engines are switched on. The engines basically work by pushing air out the back of them. The plane begins to accelerate and reaches take off speed, and takes off.

Still with me? good. Now imagine this-

A plane is on a giant treadmill, programmed to travel the same speed as the plane.

The planes engines switch on the engines, pushing air out the back, causing the plane to accelerate.

The treadmill tries to stop this, but because the wheels on the plane are free wheeling and in no way connected to the engine, they cannot stop the plane from accelerating. It doesn't matter how fast the treadmill spins, the planes wheels are free wheeling, and therefore will have little effect on how quickly the plane accelerates.

The plane will still be pushing air out the back, causing it to move foward.

What happens the instant after the plane leaves the treadmill? We've established that the plane is not moving relative to the ground, and hence the instant the plane leaves the treadmill it's not moving at all, except for some freely spinning wheels. So the plane will fall straight back onto the treadmill, because no air moves over the wings.

The engines provide forward impulse, not upwards. So we can make the plane move forwards as much as we like, but the treadmill will compensate for this by making the plane move backwards.

Duder asked for an explanation as to why a plane sitting on a moving treadmill with its engines off, will in fact move backwards. Your explanation doesn't address this question.

My brain is starting to hurt from talking to you people- maybe I caught the stupid bug that's going about.

The plane IS moving. The treadmill cannot stop the plane from moving.

An example given earlier explains it pretty well-

Imagine you are on roller blades on a treadmill. You turn the treadmill on, and you are thrown off the back.

Now imagine the same scenario again except there is a rope attached to a wall in front of you. You grab the rope- it doesn't matter what speed you set the treadmill to, you will still be able to pull yourself foward.

It doesn't matter what speed the treadmill is traveling,because the wheels on the plane are similiar to those on roller blades(free wheels) the plane will still be able to pull itself foward using the air through its jet engines/propellors.

the riddler wrote:

An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at the same speed the plane is moving. Can the plane take off?

THE PLANE WILL NOT LEAVE THE GROUND. WHAT "LIFTS" THE PLANE IS THE HIGHER AIR PRESSURE UNDER THE WINGS PUSHING THE PLANE INTO THE LOWER PRESSURED AIR OVER THE TOP OF THE WINGS. AS A PLANE MUST BE MOVING THROUGH THE AIR TO GENERATE THIS PRESSURE DIFFERANCE (GRADIENT), IT CANNOT TAKE OFF, OR FLY, AS LONG AS THE TREADMILL KEEPS THE PLANE FROM MOVING FORWARD...

Gravitation Gertie wrote:

Clarification: When the plane "starts its engines" there are two possible means of propulsion, wheel-driven and jet propulsion. Planes use both when on the ground before take-off.

Would you care to REclarify that sentence? What makes you think Planes can power the landing gear?

The only reasonable explanation for people still thinking the plane will not take off is that they're assuming the "...at the same speed the plane is moving" wording of the original riddle is saying that the plane is moving at the same speed in relation to the belt, which in that case, it would be stationary JUST BECAUSE THE RIDDLE SAYS SO. Notice the plane is always CAPABLE of moving forward in relation to the air around it regardless of either interpretation.

ok, will a "jet" plane fail to take off (no air over the wings) and a prop plane take off (air forced over the wings)?

This thread is hilarious; all of the people arguing 'til they're blue in the face that the plane won't take off. I find the fact that the aerospace guy said that it won't take off particularly disturbing. The element of the problem that makes the plane take off is that it doesn't say that the plane's speed is being measured relative to the treadmill (or by rotations of the wheels, a la a car spedometer), so the speed is being measured relative to the ground. I don't care what vehicle you're driving, if you're moving 150 miles per hour, you're moving 150 miles per hour. The only way the anti-takeoff theory would work is if the speed is being measured as a car's spedometer measures speed, by rotations of the wheels. However, that is not the case here, being that the general reference point of speed is the ground. I'll give a little credit to those arguing for no takeoff on the belief that speed is refering to "spedometer speed" or speed relative to the treadmill belt, since the problem doesn't explicitly state a reference point for speed (the ground or the belt). The ground is the best and logical assumption though.

Braavo Furgi wrote:

THE PLANE WILL NOT LEAVE THE GROUND. WHAT "LIFTS" THE PLANE IS THE HIGHER AIR PRESSURE UNDER THE WINGS PUSHING THE PLANE INTO THE LOWER PRESSURED AIR OVER THE TOP OF THE WINGS. AS A PLANE MUST BE MOVING THROUGH THE AIR TO GENERATE THIS PRESSURE DIFFERANCE (GRADIENT), IT CANNOT TAKE OFF, OR FLY, AS LONG AS THE TREADMILL KEEPS THE PLANE FROM MOVING FORWARD...

This is quite possibly the stupidest thing I have ever heard. Ever.

The Plane isn't pushed up into the air by higher air pressure under the wing, rather it is lifted up by the vaccum created on the upper side of the wing.

The bottom side of the wing is flat, the upper side being curved. The air accelerates faster over the top of the wing, creating a LOW pressure area- this has the effect of sucking the plane up into the air.

Maybe you should read this before you reply.

The name you posted under is idiot, and that is what you've proved yourself to be.

The propellors on a prop plane do not force air over the wings, they have the effect of pulling or pushing the plane foward- it depends on their location on the wing.

The faster the plane moves forward, the more air moved over&under the wings, increasing lift etc, etc.

The jet plane is doing almost the same as the prop plane- pulling the air through the engine and forcing it out the back.

I suggest you also read this before posting a reply-

Let me repeat. I was wrong earlier and it will fly. The treadmill has to be as long as a runway for it to work. And just for clarification its the speed of the treadmill belt.

My sudden epiphany, prompted by numerous "it will fly" posts, tells me the speed of the treadmill will only affect the speed the wheels spin, the engines will propell the plane forward independently. Speed up the treadmill by ten times and the wheels will spin ten times faster but the plane will move forward at the same speed.