So when your body is moving 10 mph on the platform of your legs on a treadmill going 10 mph in the opposite direction, are your legs spinning at what would on normal ground be 20 mph? No. You don't add the speeds.
So when your body is moving 10 mph on the platform of your legs on a treadmill going 10 mph in the opposite direction, are your legs spinning at what would on normal ground be 20 mph? No. You don't add the speeds.
What you assume in that example is that the wheels on the airplane will simply coast at the speed of the treadmill, before the force of the engine is taken into effect. This is not how it happens. If the engine on the airplane is never turned on, and no force is applied to the plane to move it forward, it will simply stay in place relative to the treadmill and go backward relative to the ground. The force of the engine is what gets the wheels moving, not the treadmill.
Say you take the wheels out of the equation altogether, and just lube the hell out of the bottom of the plane and the treadmill. Without force from the engine, the plane will stay put on the tread and slide back. When the force from the engine accelerates the plane, the treadmill matches the speed and the airplane stays put relative to the ground.
<-----Actual aerospace guy wrote:
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?
No I am not with you so far because you are already wrong. The speed of the skateboard has nothing to do with the speed of the treadmill. That is a false asumption you make. Think of it like this. If the treadmill were a frictionless surface then it doesn't matter how fast it moves it will have no effect on the skateboard. Well the treadmill isn't a frictionless surface but because it is acting on wheels it is close to it. The speed of the treadmill subtracts a little from the speed of the skateboard because of a small amount of friction, but not by much.
I'm trying to decipher everyones angles of thought on this. I've gone back and forth a couple times. Could someone answer this please. Say the plane is idle and the infinitely long treadmill revs up to 150 mph or knots. Now the plane would be going backwards at 150 knots, is that right? Now the plane starts revving up its engines and slowly brings them up to the point where it's no longer losing ground on the treadmill. Is it going 150 knots and if so does it takeoff from there? Thanks for the headache. :)
See the reply above. The wheels don't move because of the treadmill, they move because of the engine. If you place a skateboard on a treadmill, and turn the treadmill on, the skateboard will move back with the tread until it falls off.
Forgive me if I have missed this already and/or I am wrong. Doesn't Bernoullli's (sp) principle dictate the fact of whether the plane will lift off or not. Since that is the case, the wheels make no difference. Depending on the plane, what is the biggest concern is the wings. If the plane's engines are able to propell the plane forward enough to overcome the treadmill then it will lift off. But, since the treadmill will increase with each succesive RPM achieved by the engine, the plane will continue to rev higher until the engine redlines and siezes. So, my opinion is the plane won't fly. I have had a couple glasses of wine though, so cut me some slack.
Yes, the plane will be going 150 knots relative to the treadmill, but will not take off because relative to the air around it where it gets its lift, it's not going anywhere.
<-----Actual aerospace guy wrote:
So when your body is moving 10 mph on the platform of your legs on a treadmill going 10 mph in the opposite direction, are your legs spinning at what would on normal ground be 20 mph? No. You don't add the speeds.
first of all it's not clear what you said here because you have little ability to clearly express your thoughts. but the 20mph speed is relative to the treadmill, not to the ground. your speed relative to the ground would still be only 10mph. I SAID THAT ALREADY. i see that you didnt' refute the roller skate example however. if you are skating at 10mph on the ground and jump right over onto a treadmill going at 10mph the other way your relative speed to the treadmill would be 20mph. YOU DONT SLOW DOWN ISTANTLY. WHAT THE HELL IS WRONG WITH YOU??
Glasses of wine or not, you're still correct.
I didn't refute that example because it has nothing to do with this riddle. In this riddle, the treadmill and the airplane both start at 0 mph and the acceleration of each is constantly equal, keeping the speeds equal.
OK, to clear up the confusion.
A treadmill belt moving at any velocity will never be able to impart a force great enough to keep the plane from moving forward relative to the ground. The only thing it acts upon are the wheels, which are free to rotate. The plane will move forward! This is the assumption that makes lift possible. I do agree that if the plane could not move forward relative to the ground, it would not take off. But it can, and would!
If the treadmill was short, the plane would hop off and then lift. If it was long, it would proceed up the treadmill belt until lift-off.
The key point here is that the belt, no matter how fast it goes, can not create a force on the plane to counteract the thrust. The riddle says the speed is matched, not the force on the plane!
Alright, my opinion is that no matter how fast the plane's engine goes, the treadmill is going to correlate it's speed as well. So, the plane's wheels may be moving at about 150kts, and the treadmill as well, but the wind going over the wings will not change. Since the wind will not change, thus not creating a low pressure which allows lift, the plane will not rise. That is a really simplified answer and I'm sure Johnny NASA may take offense to my laymans description, but that is how I see it.
By the way, to the guy that started this, nice job, it is nice to actually get everyone thinking now and then. I thought I was the only one who nerds out over this stuff.
The riddle supposes that the speed of the treadmill would be linked to the speed of the plane. Until the air passing under the wings reaches a high enough speed to provide lift, it rests on the wheels, and goes the same speed as the treadmill underneath it, and goes nowhere, getting no lift.
Looks like the riddle needs a rewrite.
"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?"
If we assume the wording as it stands, then it appears the plane will move forward and take off.
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If we were to rewrite the riddle as follows...
"An airplane is sitting on an enormous treadmill. As the plane starts its engines, the treadmill runs in the opposite direction at a speed that is sufficient to keep the plane from moving. Can the plane take off?"
Then the answer would be, obviously, no.
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However, to keep that plane from moving forward the friction in the wheel system would have to be ENORMOUS and require INSANE belt speed.
No offense at all, and maybe if the others would have some wine and relax for a second they'd be able to see it as you do as well.
I think the wording is fine. I once again put forth the idea of a runner on a treadmill. If you start from 0 mph, and gradually accelerate the treadmill, a runner the keeps the same speed as the treadmill will go nowhere.
Seriously man, I'm not Lance bass, but I really want to go to the moon. Honestly, just pass it along the line, see if I can get anywhere. And looking at my post, sorry if I came across as a dick, I meant it as "NASA" as a whole. Not in a hole, as a whole. The hole thread is another entity unto itself.
One last post before turning in.
The plane's engine is not linked at all to the wheel system. The wheels rotate independent of engine RPMs, so...
If the treadmill belt matches the plane's airspeed, the plane will overcome the force of friction in its wheels, the only thing holding it back, and take off.
If the treadmill belt matches an arbitrary speed that is enough to cause friction in the wheels that holds the plane stationary, then it will not take off.
Glasses of wine or not, those are your two choices.