A harmless little riddle

Treadmill

Ok, one little point that is really of no consequence to the final answer. Everyone agrees that the plane moves because of the engines, not the wheels. Therefore, if the plane tires are still in contact with the treadmill that is going as fast as the plane (or as fast as the plane would be going), then the tires on the plane are not "spinning twice as fast". If the treadmill is moving at 150 mph, then the tires are also spinning at 150 mph.

Let me know if that makes sense...

OK, here's the part I don't get: Almost everyone agrees that the plane won't be able to take off, so how did this thread get to five pages so fast?

<-----Actual aerospace guy wrote:

That example would only work if the car were also on the treadmill. If the car is not on the treadmill, then the boy on the skateboard will be going faster than the treadmill, which is not the situation posed by the riddle.

The treadmill moves at the same speed the airplane is moving. When a treadmill moves at the same speed as a runner, the runner doesn't go anywhere. If the runner goes 10 mph, and the treadmill goes 10 mph, the runner goes nowhere relative to the air around him.

WRONG. The boy on the skateboard will be going the same speed as the treadmill. That is how the original riddle was posed. The treadmill is programmed to match the speed of whatever is on it. It is the same exact situation as the riddle.

Now answer my riddle. What happens?

Rocket yes, plane no.

If a plane could get airborne simply by the thrust of the engines (and some can; some can accelerate in a verticle climb) it would not really need wings. But if the plane needs its wings to fly, it wont go.

The riddle you pose is an impossibility. The speed of a skateboard on such a treadmill while being towed by a car will be the speed of the treadmill plus the speed of the car which means that except while the car is idle, your situation would never be possible. This has nothing to do with the speed of the treadmill matching the speed of an airplane.

Lulu wrote:

Ok, one little point that is really of no consequence to the final answer. Everyone agrees that the plane moves because of the engines, not the wheels. Therefore, if the plane tires are still in contact with the treadmill that is going as fast as the plane (or as fast as the plane would be going), then the tires on the plane are not "spinning twice as fast". If the treadmill is moving at 150 mph, then the tires are also spinning at 150 mph.

Let me know if that makes sense...

If a plane was moving at 150mph (relative to the ground) then the treadmill underneath it is moving 150mph the other way (relative to the ground). Therefore the relative speed between the plane and treadmill is 300mph.

And another thing...I don't see why the treadmill would be moving anyway. Turn up the engines and your aren't really getting the wheels to move unless the plane moves.

It's the same speed, you don't add them together. Like I said earlier, when a runner runs 10 miles per hour and the treadmill is moving at 10mph, the runner goes nowhere. There's no 20 mph relative speed between the two.

Lulu wrote:

Ok, one little point that is really of no consequence to the final answer. Everyone agrees that the plane moves because of the engines, not the wheels. Therefore, if the plane tires are still in contact with the treadmill that is going as fast as the plane (or as fast as the plane would be going), then the tires on the plane are not "spinning twice as fast". If the treadmill is moving at 150 mph, then the tires are also spinning at 150 mph.

Let me know if that makes sense...

Yes, I agree - the wheels will match the treadmill speed - so 150mph, not 300mph

But - if the wheels are spinning at 3mph, 150mph, 300mph, or 3,000,000mph - it doesn't matter - no lift=no flight.

If you would like, we can now let this thread join the Bobby Lockhart threads and others like it in the back pages of the forum. Or if you still have questions regarding why it will not be able to take off, let me know.

PS I start work for NASA on the 16th. We discussed this in more than one class. Thanks for the vote of confidence.

what I was getting at was that the fact there is a tredmill makes no difference, it is irrevelant. Everyone is spending so much time focusing on the tredmill. Like the skatebaoard on the treadmill it doesnt matter how fast the the treadmill or the wheels will go because the thrust of the engines will do the same thing as the rope tied to the bumper of the car mentioned earlier. The fact is since the wheels are free spinning and do not drive the plane, it will keep accelerating and move forward and will keep doing so til its relative airspeed is high enough to provide the amount of lift to bring it airborn.Also another way to think aobut it is the ground does not move, this does not cause a plane to immediately skid to a stop on landing

Treadmill

<-----Actual aerospace guy wrote:

The riddle you pose is an impossibility. The speed of a skateboard on such a treadmill while being towed by a car will be the speed of the treadmill plus the speed of the car which means that except while the car is idle, your situation would never be possible. This has nothing to do with the speed of the treadmill matching the speed of an airplane.

YOU DON'T EVEN MAKE SENSE!!!! What the hell are you talking about???

You're telling me that it is impossible to invent a treadmill that matches the speed of whatever is on it???? Not only is that BS but that's the same god damn thing that is supposed in the original riddle!

And I'll tell you what would happen. The boy would move forward at the speed of the car while the treadmill moved backwards at the speed of the car. This means that the wheels would spin twice as fast and the boy would fall off the front end of the treadmill and continue on.

How the hell do you think a treadmill would ever match the speed of an airplane on its own???? The airplane exerts no force against the treadmill. The treadmill would only start moving if it was programmed to match the speed of the plane. Which is exactly the same god damn thing that happens in my f***ing riddle.

Please take a different handle because it is obvious you haven't yet graduated high school, let alone gotten any sort of degree in aerospace.

The problem there is in the stating of the riddle. The treadmill goes at the same speed as the plane, matching it's velocity, thus there is no forward movement. The engine is what propels the plane forward, yes, but it is still resting on the wheels. As much as the plane accelerates, it will rest on the wheels which are spinning in place on the treadmill, and will not move forward or gain any lift.

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 answer is yes, it can take off.

How So?

The riddle never specifies that the plane HAS to take off of treadmill.

The plane could legally be taken off of the treadmill and placed on a runway would it could then take off.

Alright, I figured it out. Yes and No. But mostly yes. Yes.Let's say that a the plane needs 1 mile of runway to take off. If the treadmill is 1 mile long, the plane will take off.Say the plane is moving left at 150 knots. The treadmill is moving right at 150 knots. The wheels will spin at 300 knots. The plane will still make progress to the left and the tread will be moving to the right and the wheels will be spinning at the plane's true air speed plus the treadmill's speed. 150 + 150 = 300. The answer is no if you assume that the plane is somehow anchored to its position in space. Or if the treadmill is shorter than the regular runway length requisite for flight. It would then require a windtunnel.

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?

It is obvious you haven't passed reading comprehension or algebra. What I said was, in your "riddle" the boy on the skateboard would first be going the speed of the treadmill while the car which was towing the skateboard would be idle.

S = speed of skateboard T = speed of treadmill C = speed of car

S = T

When C = 0 (car is idle), S = T + C

With me so far?

The moment the car begins to accelerate, you are adding an outside force to the skateboard. When the car is idle, the skateboard, by staying in place, is going the same speed as the treadmill, no matter how fast the treadmill is moving. Accelerate the car, and the skateboard's speed is now the speed of the treadmill plus the speed of the car.

If you change C, the speed of the car, your riddle becomes impossible because S = T only when C = 0. If you change C, then T cannot equal S meaning the speed of the treadmill cannot equal the speed of the skateboard.

<-----Actual aerospace guy wrote:

It's the same speed, you don't add them together. Like I said earlier, when a runner runs 10 miles per hour and the treadmill is moving at 10mph, the runner goes nowhere. There's no 20 mph relative speed between the two.

YOU ARE AN IDIOT!!! When a runner is running on a treadmill at 10mph that is RELATIVE to the treadmill!!! Not to the ground! His speed relative to the ground is 0mph!!!!!!! That is why the relative speed to the treadmill is only 10mph.

Now lets say that the runner is running at 10mph right BESIDE the treadmill, not on top of it. Let's also say that the treadmill is moving by itself at 10mph the opposite direction. Can you now see that there is a relative speed differential of 20mph????????? Now lets say you plop the runner onto the treadmill as his momentum is still at 10mph (relative to the ground). What is going to happen? I'll tell you what will happen, he will quickly be slowed down to 0mph (relative to the ground) and 10 mph relative to the treadmill. That is because of the friction between his feet and the treadmill. But lets say he had on roller skates while he was moving at 10mph next to the treadmill. When you pick him up and plop him on the treadmill (with a forward speed of 10mph) those wheels are initially going to be going at 20mph and eventually slow down to 10mph. UNLESS there is a continual force applied to him that pushes him forward at 10mph (relative to the ground).

Say your buddy was on roller skates and skating on a very very long treadmill that was going backwards at 10mph. Intially the guy on roller skates, skating at 10mph is going to have a speed relative the ground of zero mph. But say you push your buddy from behind (you yourself are not on the treadmill) and you push him while running forward (running right beside the treadmill) at 10mph to the ground. Suddenly he is going to be going at 10mph to the ground as well, and with the treadmill still going at 10mph the opposite direction he will have a relative speed to it of 20mph. Even if the treadmill sped up to 20mph the opposite direction while you are still pushing your buddy at 10mph it wouldnt slow you down much, you'd still be making close to 10mph true speed while now the speed differential is 30mph.

And NASA must truly be in the shitter if it's now hiring idiots like yourself.