Delta = change
T = time
In this regard distance/time = velocity (a single change) - 1st order deriviative resulting in an integer result....a constant.
Acceleration = increase of velocity - 2nds order derviative, a change in the 1st order.
I'm sure I botched the hell out of that one but it's been 15 years since I took statics/dynamics and calculus.
vizkiz
Senior member
- Sep 20, 2005
- 216
- 0
- 0
Do you guys have .net installed on your computers?
If so, my friend wrote a program to help explain this.
Here it is. He named it, not me.
If so, my friend wrote a program to help explain this.
Here it is. He named it, not me.
Bill Brasky
Diamond Member
- May 18, 2006
- 4,324
- 1
- 0
So is this force coming from friction? I don't understand how the treadmill transfers force to the fuselage of the plane through the wheels.
Also, I thought the treadmill spins at a rate equal to the speed of the plane. I don't remember reading anything about applying an equal force.
OK, I read the whole thread. I'm amazed.
There was almost the right thing posted once ... the missing explanation:
You (non take-offers) are on a skateboard, which is located on a treadmill. You have a rope around your waist that is attached to a '57 Chevy's rear bumper.
Begin: the car is stationary. The treadmill starts up moving however fast you want it to go. The rope tightens and you remain stationary on the treadmill, relative to the surrounding air. The skateboard wheels match the speed of the treadmill belt.
Later: The greaser sitting at the wheel of the '57 Chevy (327, blower, pipes, upper/lower work) pops the clutch and zooms off to the burger stand. The rope will pull you off the front of the treadmill (trust me). Even if the treadmill speeds up to match the speed of the wheels on your skateboard, the rope (which would correspond to the prop of an airplane as a motivational force) is going to pull you forward and off the front of the treadmill. Your "airspeed" - how fast you are going relative to the surrounding air - will increase.
Later still: The Greaser gets a cheesburger, fires, and malt *and* a date with Suzy the Cheerleader.... and you end up looking like Grandma's dog in Lampoon's Summer Vacation movie.
Disclaimer: *Never* tie yourself to the bumper of a car, especially a '57 Chevy driven by a hungry Greaser, with out without a skateboard ...
FWIW
There was almost the right thing posted once ... the missing explanation:
You (non take-offers) are on a skateboard, which is located on a treadmill. You have a rope around your waist that is attached to a '57 Chevy's rear bumper.
Begin: the car is stationary. The treadmill starts up moving however fast you want it to go. The rope tightens and you remain stationary on the treadmill, relative to the surrounding air. The skateboard wheels match the speed of the treadmill belt.
Later: The greaser sitting at the wheel of the '57 Chevy (327, blower, pipes, upper/lower work) pops the clutch and zooms off to the burger stand. The rope will pull you off the front of the treadmill (trust me). Even if the treadmill speeds up to match the speed of the wheels on your skateboard, the rope (which would correspond to the prop of an airplane as a motivational force) is going to pull you forward and off the front of the treadmill. Your "airspeed" - how fast you are going relative to the surrounding air - will increase.
Later still: The Greaser gets a cheesburger, fires, and malt *and* a date with Suzy the Cheerleader.... and you end up looking like Grandma's dog in Lampoon's Summer Vacation movie.
Disclaimer: *Never* tie yourself to the bumper of a car, especially a '57 Chevy driven by a hungry Greaser, with out without a skateboard ...
FWIW
Here's the answer for ALL of you:
Click me!
I'm confident that's the right solution.
Here's the gist of it:
According to Paul J. Camp, a professor in the department of physics at Spelman College, it's all pretty simple. "At first, the conveyor will hold the plane still. But only to a certain point, after which, driven by thrust from its engines, the craft will accelerate."
But the problem clearly states: The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction.
"The key is in the behavior of friction," Camp says. "Friction is a peculiar force in that it has an upper limit. For instance, push an object on your desk, but not hard enough to move it. Why doesn't it move? Because the friction force exactly balances the force of your push. At some point you push hard enough to set the object in motion. This is the point where friction has topped out and is not capable of growing any larger."
With the airplane and treadmill, there is, at the outset, friction force capable of rotating the tires at the proper speed to keep the plane stationary. However, as the thrust is increased, that force eventually maxes out. (Two separate frictions are at play here, actually, one between the tires and belt, the other between the plane's axles/bearings and its wheels. The first will max out before the second.)
"And at that point the wheels no longer roll, they slide," says Camp. "Or rather, they roll and slide at the same time. Tire motion is now decoupled from the belt motion. No matter how much you whiz up the treadmill, you won't add any more rotational velocity to the wheels because friction is already doing everything it is capable of. The plane skids toward takeoff -- likely accompanied by much smoke and a powerful rubbery stink."
And there you have it, at least on paper. Bear in mind that for a plane to reach that point of decoupling would require two things above and beyond the pale of normal engineering. First, a remarkable amount of power -- far more than any jetliner, and probably any military plane, is capable of developing. The illustration on Pogue's blog is of an Airbus A320; some sort of rocket plane would be more appropriate. Second, no existing aircraft tires could take such abuse. The rotational velocity required before reaching the friction limit would have them bursting within seconds, causing the plane to be flung backward. Believe it or not, landing gear isn't engineered with giant treadmills in mind, and pilots need to adhere to maximum groundspeed limits, lest their tires wind up like this. These limits occasionally present problems during tailwind operations or in the case of flap and slat malfunctions -- scenarios dictating the need for unusually high takeoff or landing speeds.
For good measure, the treadmill itself, as described, could never be built. It can't "exactly match the speed of the wheels," because the wheels will turn at the speed of the treadmill plus the speed of the plane relative to the ground. When the speed of the plane is greater than zero (which it is the moment its wheels start to spin; otherwise they would never move), then the problem becomes impossible. By definition, the wheels have to be turning faster than the treadmill.
Click me!
I'm confident that's the right solution.
Here's the gist of it:
According to Paul J. Camp, a professor in the department of physics at Spelman College, it's all pretty simple. "At first, the conveyor will hold the plane still. But only to a certain point, after which, driven by thrust from its engines, the craft will accelerate."
But the problem clearly states: The conveyer belt is designed to exactly match the speed of the wheels, moving in the opposite direction.
"The key is in the behavior of friction," Camp says. "Friction is a peculiar force in that it has an upper limit. For instance, push an object on your desk, but not hard enough to move it. Why doesn't it move? Because the friction force exactly balances the force of your push. At some point you push hard enough to set the object in motion. This is the point where friction has topped out and is not capable of growing any larger."
With the airplane and treadmill, there is, at the outset, friction force capable of rotating the tires at the proper speed to keep the plane stationary. However, as the thrust is increased, that force eventually maxes out. (Two separate frictions are at play here, actually, one between the tires and belt, the other between the plane's axles/bearings and its wheels. The first will max out before the second.)
"And at that point the wheels no longer roll, they slide," says Camp. "Or rather, they roll and slide at the same time. Tire motion is now decoupled from the belt motion. No matter how much you whiz up the treadmill, you won't add any more rotational velocity to the wheels because friction is already doing everything it is capable of. The plane skids toward takeoff -- likely accompanied by much smoke and a powerful rubbery stink."
And there you have it, at least on paper. Bear in mind that for a plane to reach that point of decoupling would require two things above and beyond the pale of normal engineering. First, a remarkable amount of power -- far more than any jetliner, and probably any military plane, is capable of developing. The illustration on Pogue's blog is of an Airbus A320; some sort of rocket plane would be more appropriate. Second, no existing aircraft tires could take such abuse. The rotational velocity required before reaching the friction limit would have them bursting within seconds, causing the plane to be flung backward. Believe it or not, landing gear isn't engineered with giant treadmills in mind, and pilots need to adhere to maximum groundspeed limits, lest their tires wind up like this. These limits occasionally present problems during tailwind operations or in the case of flap and slat malfunctions -- scenarios dictating the need for unusually high takeoff or landing speeds.
For good measure, the treadmill itself, as described, could never be built. It can't "exactly match the speed of the wheels," because the wheels will turn at the speed of the treadmill plus the speed of the plane relative to the ground. When the speed of the plane is greater than zero (which it is the moment its wheels start to spin; otherwise they would never move), then the problem becomes impossible. By definition, the wheels have to be turning faster than the treadmill.
Mo0o
Lifer
- Jul 31, 2001
- 24,227
- 3
- 76
People that say the plane wont take off are thinking of the plane like a car but fail to realize theres a big difference between getting acceleration from the wheels and from an jet engine. it is theoritically possible to keep a car stationary on a treadmill, not so with a plane
No. At least this "non-taker-offer" completely understands that. Makes perfect sense and obeys physics.
This situation however does not and therefore has no answer. Stop thinking real world and think how the scenario is presented. This isn't real world stuff.
exdeath
Lifer
- Jan 29, 2004
- 13,679
- 10
- 81
Ok I'm going to try one more time...
You can try this at home if you want.
1) First tie a rope between two trees.
2) Place a treadmill parallel to the orientation of the rope such that the rope is in reach when standing on the treadmill.
3) Stand on the treadmill on a skateboard.
3) Now slowly (because you don't have much length on a typical treadmill) pull yourself against the treadmill using the rope, starting at the bottom of the treadmill.
4) While you do this, have a friend gradually increase the speed of the treadmill.
5) Observe that you can still pull on the rope and propel yourself against the treadmill with little effort, the only effect being that the wheels on the skateboard are moving faster than normal.
Why is this valid? Jet thrust works on Newton?s third law: an object that exerts a force on another object experiences a reaction force equal in magnitude and opposite in direction of the applied force. I.e.: for every action there is an equal and opposite reaction, in high school parlance.
Jet engines propel air particles in one direction, thus experience an equal force in the opposite direction, in effect pushing or pulling against the air. This is similar to the rope. By applying a force on the rope and pulling it in towards you and pushing it behind you, the reaction force results in your going forward.
Like the rope, the air provides an available external stationary medium upon which to apply a constant force against in order to achieve that forward propelling reaction force. This force is completely independent of the treadmill/wheel interactions, in exactly the same way you are able to pull yourself against the movement of a treadmill using a rope as described, or stationary railing beside the treadmill, etc.
*holds breath*
6) optional: if you still don't get it, use the rope to hang yourself instead.
You can try this at home if you want.
1) First tie a rope between two trees.
2) Place a treadmill parallel to the orientation of the rope such that the rope is in reach when standing on the treadmill.
3) Stand on the treadmill on a skateboard.
3) Now slowly (because you don't have much length on a typical treadmill) pull yourself against the treadmill using the rope, starting at the bottom of the treadmill.
4) While you do this, have a friend gradually increase the speed of the treadmill.
5) Observe that you can still pull on the rope and propel yourself against the treadmill with little effort, the only effect being that the wheels on the skateboard are moving faster than normal.
Why is this valid? Jet thrust works on Newton?s third law: an object that exerts a force on another object experiences a reaction force equal in magnitude and opposite in direction of the applied force. I.e.: for every action there is an equal and opposite reaction, in high school parlance.
Jet engines propel air particles in one direction, thus experience an equal force in the opposite direction, in effect pushing or pulling against the air. This is similar to the rope. By applying a force on the rope and pulling it in towards you and pushing it behind you, the reaction force results in your going forward.
Like the rope, the air provides an available external stationary medium upon which to apply a constant force against in order to achieve that forward propelling reaction force. This force is completely independent of the treadmill/wheel interactions, in exactly the same way you are able to pull yourself against the movement of a treadmill using a rope as described, or stationary railing beside the treadmill, etc.
*holds breath*
6) optional: if you still don't get it, use the rope to hang yourself instead.
The plane struggles to fly, then pitches up before leveling off and becoming airborne. Then it suddenly losses altitude and crashes into this thread destroying all of the posts...
A VTOL plane's ability to take off vertically has nothing to do with the surface underneath the plane. You could be at 3000 feet and hover if you wanted to.
That conundrum isn't even a conundrum. That's just idiocy at work. the lift comes from air, if no air is moving how can a plane lift off? It might work with propeller planes since the propellers move air under/around the wings to make the plane move forward, but in a jet airplane there is just no way.
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