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re: Can this 747 take off?
Posted on 4/11/24 at 8:50 pm to NATidefan
Posted on 4/11/24 at 8:50 pm to NATidefan
quote:
A plane with wheels can't advance down a runway unless the wheels spin
When car brakes lock up, can the car skid? Or does the car just stop that instant.
With enough thrust, the friction on the wheels is just a weak memory.
747s have a lot of thrust, by the way. 240,000 horsepower.
But hey... there is good tread on those tires. I'm sure they will put up a helluva fight holding that jet plane still.
1
Posted on 4/11/24 at 8:51 pm to loogaroo
The thrust from the engines is significantly larger than any force the treadmill could exert on the wheels. If a plane's wheels operated like a car's wheels, what y'all are saying makes more sense. But we all know that's not how it works.
For the plane to stay stationary would require picking and choosing which aspects of physics to ignore on top of the magical conveyor belt existing.
For the plane to stay stationary would require picking and choosing which aspects of physics to ignore on top of the magical conveyor belt existing.
Posted on 4/11/24 at 8:51 pm to LNCHBOX
The treadmill is decelerating in the gif, all else equal
Posted on 4/11/24 at 8:53 pm to Power-Dome
quote:
The treadmill is decelerating in the gif, all else equal
I quit
Yall are beyond help. Posted on 4/11/24 at 8:55 pm to LNCHBOX
quote:
The thrust from the engines is significantly larger than any force the treadmill could exert on the wheels.
Yes that’s why it is impossible, but for the sake of the riddle, the treadmill is able to go this fast to create the force required to counteract the thrust of 4 jet engines. That’s why I wrote the thought experiment about the small balsa wood plane with little thrust required for airborne
quote:
I quit Yall are beyond help.
You asked how it was possible
This post was edited on 4/11/24 at 8:57 pm
Posted on 4/11/24 at 9:01 pm to NATidefan
quote:
All I know is them wheels better be spinning as fast as that tread mill.
Nope
Posted on 4/11/24 at 9:02 pm to Power-Dome
quote:
You asked how it was possible
The speed of that treadmill is constant
Posted on 4/11/24 at 9:07 pm to LNCHBOX
It is not level
Posted on 4/11/24 at 9:07 pm to meansonny
quote:
When car brakes lock up, can the car skid? Or does the car just stop that instant.
It skids to a stop, decelerating the whole time.
quote:
With enough thrust, the friction on the wheels is just a weak memory.
Well the wheels would just spin faster and faster until they burned up. Then you have metal on treadmill. Like a 747 trying to take off without wheels grinding down a runway. I'm sure that'll work out fine.
Like the other poster said if they all hold up equally the plane goes nowhere. If the treadmill breaks first or couldnt keep up the plane takes off. If the tires go or engines lose out first it doesn't.
But the treadmill is the only thing that is said can equally match the speed of the wheel rotation in the question. So it's the inifinte one in this case.
This post was edited on 4/11/24 at 9:09 pm
Posted on 4/11/24 at 9:08 pm to Power-Dome
yep this thread is done. 
Posted on 4/11/24 at 9:13 pm to NATidefan
You are late to the party.
It is physically impossible for the treadmill to match the speed of the jet planes freewheels.
The reference in the thread is "a magic treadmill". Because it would take magic on the treadmill or some sort of regulator on the plane's wheels to keep them in sync.
And if there is a regulator on the plane's wheels to keep it from spinning too fast (so it can keep pace with the treadmill), the jet propulsion is just going to push the wheels forward anyway.
It is physically impossible for the treadmill to match the speed of the jet planes freewheels.
The reference in the thread is "a magic treadmill". Because it would take magic on the treadmill or some sort of regulator on the plane's wheels to keep them in sync.
And if there is a regulator on the plane's wheels to keep it from spinning too fast (so it can keep pace with the treadmill), the jet propulsion is just going to push the wheels forward anyway.
Posted on 4/11/24 at 9:14 pm to Street Hawk
I can’t believe this thread. Let me help out everyone who said no, because they don’t understand the physics or those who said no because of the wording of the question. The answer is yes even within the confines of the question, which is what makes it a trick question.
The question says the conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. The answer is that it’s impossible to achieve this regardless of design. Which is what makes this an engineering brain teaser.
Why is this?
Assume the speed of the conveyor is X, while sitting with no engine thrust the wheels speed can also be described as X, and that formula is X=X (speed of the belt on the left, speed of the wheels on the right) which people said cancels motion because the directional vectors are opposite. Many people who said no stopped here.
Now assume the forward momentum from the engine thrust as Y. When the engines power up the formula changes to X=X+Y. So long as Y isn’t zero this formula becomes mathematically impossible at any value of X. This is as a dependent (x) and independent (y) variable.
So regardless of what speed the conveyor is going it can never ever match the speed of the wheels as the wheels speed is the speed of the belt plus the additional thrust.
So as you approach infinity, assuming nothing would mechanically fail, the plane not only takes off, but it does so as if it was sitting on concrete.
The question is designed to show people studying engineering that some things are mathematically impossible to engineer.
The question says the conveyor belt is designed to exactly match the speed of the wheels, moving in the opposite direction. The answer is that it’s impossible to achieve this regardless of design. Which is what makes this an engineering brain teaser.
Why is this?
Assume the speed of the conveyor is X, while sitting with no engine thrust the wheels speed can also be described as X, and that formula is X=X (speed of the belt on the left, speed of the wheels on the right) which people said cancels motion because the directional vectors are opposite. Many people who said no stopped here.
Now assume the forward momentum from the engine thrust as Y. When the engines power up the formula changes to X=X+Y. So long as Y isn’t zero this formula becomes mathematically impossible at any value of X. This is as a dependent (x) and independent (y) variable.
So regardless of what speed the conveyor is going it can never ever match the speed of the wheels as the wheels speed is the speed of the belt plus the additional thrust.
So as you approach infinity, assuming nothing would mechanically fail, the plane not only takes off, but it does so as if it was sitting on concrete.
The question is designed to show people studying engineering that some things are mathematically impossible to engineer.
This post was edited on 4/11/24 at 9:17 pm
Posted on 4/11/24 at 9:24 pm to meansonny
quote:
You are late to the party.
It is physically impossible for the treadmill to match the speed of the jet planes freewheels.
The reference in the thread is "a magic treadmill". Because it would take magic on the treadmill or some sort of regulator on the plane's wheels to keep them in sync.
And if there is a regulator on the plane's wheels to keep it from spinning too fast (so it can keep pace with the treadmill), the jet propulsion is just going to push the wheels forward anyway.
I don't care if it's impossible or not, that's not the way the question is stated. My magic treadmill that is infinitely fast, can instantly match speed, and as long as a runway exists in this scenario. The 747 and its wheels are just run of the mill.
This post was edited on 4/11/24 at 9:25 pm
Posted on 4/11/24 at 9:26 pm to Power-Dome
The riddle isn’t does the plane move it’s that it’s impossible for a conveyor to be designed to match the speed of the wheels. The wheels speed is the speed of the belt plus the additional speed of the trust, so be it 1 mph or infinity the speed of the wheels will always be higher.
Posted on 4/11/24 at 9:27 pm to NATidefan
quote:
that's not the way the question is stated
It doesn't matter how the question is stated.
The friction of the treadmill isn't going to have any effect on 240,000 horsepower thrusting forward.
Posted on 4/11/24 at 9:27 pm to PhysicsGuy
Except x is a velocity and y is a force in your case, your units aren’t equal
Posted on 4/11/24 at 9:31 pm to NATidefan
It can’t instantly match it, that’s the point. No one seemed to read the question correctly and got hung up on the plane. The riddle is no belt can be designed to match the wheels because the wheels speed is the speed of the belt plus an independent source of forward motion. Meaning the wheels always move faster by whatever speed the independent source is imparting.
Speed of the wheels = speed of belt+speed from thrust. So you can put any number in for speed of the belt and it’s still gonna be slower than the wheels.
Speed of the wheels = speed of belt+speed from thrust. So you can put any number in for speed of the belt and it’s still gonna be slower than the wheels.
Posted on 4/11/24 at 9:32 pm to PhysicsGuy
How is wheel speed going to be different than any other part of the plane
Posted on 4/11/24 at 9:34 pm to Power-Dome
That’s irrelevant because at the wheel the thrust is converted to forward velo. The point is if z is x plus y, so long as y is not zero x can never equal z, at any number.
Posted on 4/11/24 at 9:36 pm to Power-Dome
No one said they would be. They would be different than the belt.
I mean you can Google this if you don’t want to believe me. The answer is it moves and not only that it moves as if the belt wasn’t even there.
I mean you can Google this if you don’t want to believe me. The answer is it moves and not only that it moves as if the belt wasn’t even there.
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