that's what i keep thinking. the belt will always move in the same exact speed as the wheels.
i think we can simplify the question and say that, a plane is moving at 10mph and the conveyor belt is moving in reverse at 10mph, will the plane take off?
that's what i keep thinking. the belt will always move in the same exact speed as the wheels.
i think we can simplify the question and say that, a plane is moving at 10mph and the conveyor belt is moving in reverse at 10mph, will the plane take off?
Can someone answer this in terms of conservation of momentum though.
If you have a rocket engine on a plane and the same situation. You're expelling gas out backwards.... but where is the forward motion.
If you have a rocket engine on a plane and the same situation. You're expelling gas out backwards.... but where is the forward motion.
Maybe it will help people to think of it this way.
A plane that has just touched down has a wheel speed = air speed. If its on a belt and the belt begins to move.
Assume at touch down belt = 0, plane = 75kts.
If the belt beings to move at 5 kts in the same direction the plane is moving the plane will not increase in speed but the wheels will spin faster. Since the wheels are on bearings there is nothing to transfer their rotation to the planes motion.
Now assume you land on a belt moving at 75kts doing 75kts in the plane, with the breaks locked up. If the belt increases speed so will the plane (assuming the tires maintain traction).
If the belt continues to increase in speed and you release the breaks the plane will begin to lose airspeed (due to drag). As this occurs the tires will begin to spin isolating the plane from the ground.
Apply this to take off and the tires/bearings ISOLATE the plane from the motion of the belt. The plane will move forward and takeoff in the same distance it usually does.
A plane that has just touched down has a wheel speed = air speed. If its on a belt and the belt begins to move.
Assume at touch down belt = 0, plane = 75kts.
If the belt beings to move at 5 kts in the same direction the plane is moving the plane will not increase in speed but the wheels will spin faster. Since the wheels are on bearings there is nothing to transfer their rotation to the planes motion.
Now assume you land on a belt moving at 75kts doing 75kts in the plane, with the breaks locked up. If the belt increases speed so will the plane (assuming the tires maintain traction).
If the belt continues to increase in speed and you release the breaks the plane will begin to lose airspeed (due to drag). As this occurs the tires will begin to spin isolating the plane from the ground.
Apply this to take off and the tires/bearings ISOLATE the plane from the motion of the belt. The plane will move forward and takeoff in the same distance it usually does.
THE PLANE DOES MOVE FORWARD ON THE CONVEYOR BELT. IT DOES NOT STAY STILL. A THREADMILL ACTING ON FREE SPINNING WHEELS CAN NOT COUNTERACT THE THRUST OF THE ENGINES AND THE WHEELS WILL ONLY SPIN FASTER AND FASTER.
Assume there is no friction in the wheel bearings. Start to move the belt without moving the plane. The plane will stay in the same place as the wheels begin to spin. Now apply power to the planes engins. The plane will move forward as the wheels spin backwards.
http://www.straightdope.com/columns/060203.html
http://www.straightdope.com/columns/060303.html
Find something better to do than insulting someone's education
If the plane can fly at 10mph it will take off.
The treadmill moving backwords doesn't matter. The wheels isolate the body of the plane from the treadmill. The wheels will be rotating BACKWARDS at 20mph.
No, airplanes require the flow of air over the wings to create lift.
Edit: SHould've read the thread first. Interesting.
Edit: SHould've read the thread first. Interesting.
The plane moves forward with RESPECT TO THE TREADMILL. If your reference frame "S" is in the treadmill being your ground, you will be moving. In this case, an object in reference frame S that is not moving (i.e. the treadmill is going backwards and the plane is glued to the treadmill), the object experiences airflow in the REVERSE direction. Now we know that S is moving backwards and the object is moving forward. Airflow is 0.
Try reference frame S' on the ground. The plane is not moving with respect to an observer on the ground, but the plane is moving with respect to the treadmill. However, AIR is not moving with respect to S', and nor is the plane. Thus air is not moving with respect to reference frame S'' (the plane). Thus there is no freaking airflow over the wings and thus there is no takeoff.
Relativity my friend. QED
I don't know where you graduated from but if you read the problem, the treadmill moves as fast as the wheels do. There is no motion. You cant even throw out ANY numbers at all.
LMAO - in other news the BBC reports a loud humming noise from Westminster Abbey. Speculation is that it is Newton spinning in his grave.
If the plane is tied down and you start to move the belt the wheels will spin. If there is 0 friction in the bearings you can release the tie downs and the wheels will still spin and the plane will remain stationary. If you apply power the plane will begin to move forwards as the wheels continue to counter act the force of the belt even if the belt increases in speed. The belt could be moving at 500mph and the plane in the oposite directionat 100mph. THE BELT SPEED IS IRREVELANT TO THE PLANE SPEED AS THE WHEELS ISOLATE THE PLANE FROM THE BELT!
TankGuys
Golden Member
- Jun 3, 2005
- 1,080
- 0
- 0
The bottom line here is that it depends on the wording of the question, and how people interpret it. There are some "wordings" where the plane won't take off, and some that the plane will take off.
If people make the strict assuption that the treadmill moves backwards at the same speed as the wheels rotate forward, you run into issues of infinite acceleration and other fun stuff, and the plane won't take off.
If people assume the belt moves backward at a velocity equal to and opposite the plane's speed, then the plane takes off just fine, wheels spinning 2x the normal speed.
Consider this thought experiment:
You know those toy cars that you roll backwards, and when you let go, they move forward? Let's say you put it on a STILL treadmill. The car moves forward fine right? Now, put said car on treadmill that's moving backwards, and we all know the car moves at it's speed minus the speed of the treadmill, right? This is because the propulsion system of the car depends on the spinning of its wheels, which are affected by the treadmill. This all makes sense so far, yes?
Now, consider a standard matchbox car. Put that car on a non-moving treadmill. What happens? It stays still. Now, turn the treadmill on, and what happens? Car rolls backwards, right? Now, reach out your hand and hold the car still, while the readmill is moving. What happens now? The car stays put right? Do you think it takes much force to actually hold it still? (try it!) NOW... try pushing the car forward, while the treadmill is moving. You can do it, right? Is it hard? Is it noticably harder than pushing the car forward when the treadmill is not moving? (again, try it!)
This is exactly what happens with a plane. The force of the engines is pushing on the plane - NOT the treadmill. Aside from the bit of friction at the axle that the engine thrust has to overcome, the engines, air, and plane don't really care what happens to the treadmill.
Seriously, anyone who has a treadmill and a matchbox car at home can easily prove this to themselves
If people make the strict assuption that the treadmill moves backwards at the same speed as the wheels rotate forward, you run into issues of infinite acceleration and other fun stuff, and the plane won't take off.
If people assume the belt moves backward at a velocity equal to and opposite the plane's speed, then the plane takes off just fine, wheels spinning 2x the normal speed.
Consider this thought experiment:
You know those toy cars that you roll backwards, and when you let go, they move forward? Let's say you put it on a STILL treadmill. The car moves forward fine right? Now, put said car on treadmill that's moving backwards, and we all know the car moves at it's speed minus the speed of the treadmill, right? This is because the propulsion system of the car depends on the spinning of its wheels, which are affected by the treadmill. This all makes sense so far, yes?
Now, consider a standard matchbox car. Put that car on a non-moving treadmill. What happens? It stays still. Now, turn the treadmill on, and what happens? Car rolls backwards, right? Now, reach out your hand and hold the car still, while the readmill is moving. What happens now? The car stays put right? Do you think it takes much force to actually hold it still? (try it!) NOW... try pushing the car forward, while the treadmill is moving. You can do it, right? Is it hard? Is it noticably harder than pushing the car forward when the treadmill is not moving? (again, try it!)
This is exactly what happens with a plane. The force of the engines is pushing on the plane - NOT the treadmill. Aside from the bit of friction at the axle that the engine thrust has to overcome, the engines, air, and plane don't really care what happens to the treadmill.
Seriously, anyone who has a treadmill and a matchbox car at home can easily prove this to themselves
The wheels DO NOT TRANSFER MOTION!
Leave the plane at idle and move the treadmill. Since there is 0 friction in the bearings the plane will not move and the wheels will begin to spin backwards. The speed of the belt doesn't matter.
Originally posted by: TankGuys
The bottom line here is that it depends on the wording of the question, and how people interpret it. There are some "wordings" where the plane won't take off, and some that the plane will take off.
If people make the strict assuption that the treadmill moves backwards at the same speed as the wheels rotate forward, you run into issues of infinite acceleration and other fun stuff, and the plane won't take off.
If people assume the belt moves backward at a velocity equal to and opposite the plane's speed, then the plane takes off just fine, wheels spinning 2x the normal speed.
Consider this thought experiment:
You know those toy cars that you roll backwards, and when you let go, they move forward? Let's say you put it on a STILL treadmill. The car moves forward fine right? Now, put said car on treadmill that's moving backwards, and we all know the car moves at it's speed minus the speed of the treadmill, right? This is because the propulsion system of the car depends on the spinning of its wheels, which are affected by the treadmill. This all makes sense so far, yes?
Now, consider a standard matchbox car. Put that car on a non-moving treadmill. What happens? It stays still. Now, turn the treadmill on, and what happens? Car rolls backwards, right? Now, reach out your hand and hold the car still, while the readmill is moving. What happens now? The car stays put right? Do you think it takes much force to actually hold it still? (try it!) NOW... try pushing the car forward, while the treadmill is moving. You can do it, right? Is it hard? Is it noticably harder than pushing the car forward when the treadmill is not moving? (again, try it!)
This is exactly what happens with a plane. The force of the engines is pushing on the plane - NOT the treadmill. Aside from the bit of friction at the axle that the engine thrust has to overcome, the engines, air, and plane don't really care what happens to the treadmill.
Seriously, anyone who has a treadmill and a matchbox car at home can easily prove this to themselves
this is the only completely accurate post
Originally posted by: TankGuys
The bottom line here is that it depends on the wording of the question, and how people interpret it. There are some "wordings" where the plane won't take off, and some that the plane will take off.
If people make the strict assuption that the treadmill moves backwards at the same speed as the wheels rotate forward, you run into issues of infinite acceleration and other fun stuff, and the plane won't take off.
If people assume the belt moves backward at a velocity equal to and opposite the plane's speed, then the plane takes off just fine, wheels spinning 2x the normal speed.
Consider this thought experiment:
You know those toy cars that you roll backwards, and when you let go, they move forward? Let's say you put it on a STILL treadmill. The car moves forward fine right? Now, put said car on treadmill that's moving backwards, and we all know the car moves at it's speed minus the speed of the treadmill, right? This is because the propulsion system of the car depends on the spinning of its wheels, which are affected by the treadmill. This all makes sense so far, yes?
Now, consider a standard matchbox car. Put that car on a non-moving treadmill. What happens? It stays still. Now, turn the treadmill on, and what happens? Car rolls backwards, right? Now, reach out your hand and hold the car still, while the readmill is moving. What happens now? The car stays put right? Do you think it takes much force to actually hold it still? (try it!) NOW... try pushing the car forward, while the treadmill is moving. You can do it, right? Is it hard? Is it noticably harder than pushing the car forward when the treadmill is not moving? (again, try it!)
This is exactly what happens with a plane. The force of the engines is pushing on the plane - NOT the treadmill. Aside from the bit of friction at the axle that the engine thrust has to overcome, the engines, air, and plane don't really care what happens to the treadmill.
Seriously, anyone who has a treadmill and a matchbox car at home can easily prove this to themselves
QFT /Thread
Originally posted by: TankGuys
The bottom line here is that it depends on the wording of the question, and how people interpret it. There are some "wordings" where the plane won't take off, and some that the plane will take off.
If people make the strict assuption that the treadmill moves backwards at the same speed as the wheels rotate forward, you run into issues of infinite acceleration and other fun stuff, and the plane won't take off.
If people assume the belt moves backward at a velocity equal to and opposite the plane's speed, then the plane takes off just fine, wheels spinning 2x the normal speed.
Consider this thought experiment:
You know those toy cars that you roll backwards, and when you let go, they move forward? Let's say you put it on a STILL treadmill. The car moves forward fine right? Now, put said car on treadmill that's moving backwards, and we all know the car moves at it's speed minus the speed of the treadmill, right? This is because the propulsion system of the car depends on the spinning of its wheels, which are affected by the treadmill. This all makes sense so far, yes?
Now, consider a standard matchbox car. Put that car on a non-moving treadmill. What happens? It stays still. Now, turn the treadmill on, and what happens? Car rolls backwards, right? Now, reach out your hand and hold the car still, while the readmill is moving. What happens now? The car stays put right? Do you think it takes much force to actually hold it still? (try it!) NOW... try pushing the car forward, while the treadmill is moving. You can do it, right? Is it hard? Is it noticably harder than pushing the car forward when the treadmill is not moving? (again, try it!)
This is exactly what happens with a plane. The force of the engines is pushing on the plane - NOT the treadmill. Aside from the bit of friction at the axle that the engine thrust has to overcome, the engines, air, and plane don't really care what happens to the treadmill.
Seriously, anyone who has a treadmill and a matchbox car at home can easily prove this to themselves
excellent example.
Think of a treadmill going downhill. The force cause by gravity simulates the engines force. The treadmill can rotate all it wan't backward but that won't do anything to stop the acceleration of a toy car.(wheels are not tied to power on toy car)
Slacker
Diamond Member
- Oct 9, 1999
- 8,623
- 33
- 91
Nice, I have never heard this one before
The wording of the question allows for lots of assumptions and conclusion jumping. There are also details left out, like the conveyor being just as long as a runway would be, regardless, the plane propels itself forward until lift is created and the plane lifts off.
The wording of the question allows for lots of assumptions and conclusion jumping. There are also details left out, like the conveyor being just as long as a runway would be, regardless, the plane propels itself forward until lift is created and the plane lifts off.
mugs
Lifer
- Apr 29, 2003
- 48,920
- 46
- 91
Originally posted by: Slacker
Nice, I have never heard this one before
The wording of the question allows for lots of assumptions and conclusion jumping. There are also details left out, like the conveyor being just as long as a runway would be, regardless, the plane propels itself forward until lift is created and the plane lifts off.
All you need is the information in the problem and an understanding of physics. There are no assumptions or conclusions to be jumped to except incorrect ones.
This is the stupidest, most pointless questions I've ever seen. I think the biggest confusion is that most people get the idea that the conveyor belt is similar to a gym treadmill. If someone would have just explained that the conveyor belt runway would have to be just as long as a regular runway, most people would understand that it would take off. It seems that the people saying it won't take off read the question as the airplane is not moving with respect to the ground next to the conveyor (as I also first thought).
edit* unless someone did explain this and I just missed it reading through this thread.
edit* unless someone did explain this and I just missed it reading through this thread.
CallMeJoe
Diamond Member
- Jul 30, 2004
- 6,938
- 5
- 81
As the aircraft applies thrust, it acquires airspeed and groundspeed simultaneously, moving forward relative to the entire conveyor assembly, not just the belt, as its tires start to turn. For the belt to turn "in reverse exactly as fast as the wheels move forward", the conveyor assembly would have to move forward with the same airspeed as the aircraft. The aircraft would lift off the belt as soon as it gained enough airspeed.
tankguys i have an treadmill at home and will experiement with this in a minute. but i think you're forgetting that the conveyor belt can change speeds. it will match the speed of the wheels.
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