Hi Max,
You made a major error here.
It is that the belt is not fixed to the ground, and the force is applied to the surrounding air. Not the ground, wheels or belt. (gears in your case).
Therefore, the plane is free to move through the air, with respect to the ground.
-Chris
Edit:spelling. X post with Cal. We agree at least.
You made a major error here.
It is that the belt is not fixed to the ground, and the force is applied to the surrounding air. Not the ground, wheels or belt. (gears in your case).
Therefore, the plane is free to move through the air, with respect to the ground.
-Chris
Edit:spelling. X post with Cal. We agree at least.
Some of us will remember the days of the early computer games.
One of them was Tim aka ‘The Incredible Machine’. A very educational and for those days advanced game with beautiful graphics.
It was a simulator to build all kind of machines and I bet the trick with the airplane should be replicable. I believe the earliest version is about 15 years old and can be found on the net.
Happy building.
/Hugo
One of them was Tim aka ‘The Incredible Machine’. A very educational and for those days advanced game with beautiful graphics.
It was a simulator to build all kind of machines and I bet the trick with the airplane should be replicable. I believe the earliest version is about 15 years old and can be found on the net.
Happy building.
/Hugo
OK to all you guys that say the speed of the wheels is irrelevant I agree if you are talking about the speed forward from the reference point of the ground. the wheel will move forward at speed x the belt will move backward at speed x, the wheel will rotate faster than normal and the The plane will take off 🙂
For those who are interpreting speed as the rotational speed of the wheel, and the belt matches this rotational speed (though as maxro points out with his gear analogy the belt speed can't actually be equal, but if you convert from angular velocity to linear, you can get the equivalent speed) then if the belt matches this speed then the plane would have to be remaining stationary from the refference point of the ground and would attain no airspeed.... this is of course not physically possible due to the propulsion mechanism. but never the less the question in no way mentions that standard physics applies.
SO this is why IMO we have some saying it will take off and some saying it wont. IT all comes down to how you interpret SPEED and relative motion, not whether you know how a jet engine works or not 😉
yes the speed of the wheels in reality has nothing to do with whether the plane takes off or not, but within the context of the question depending on how you interpret it, it has everything to do with it!!
IT is an extremely poor question.
I have an uncle who at uni got the question
you are on the roof of a building and have a theotelite how would you determine the height of the building?
he said he would drop the theotelite off the building and time its decent and based on gravity being 9.8M/s/s he would calculate the height of the building. He was marked wrong (probably for being a smartass) but when he protested that the question didn't stipulate how the theotelite was to be used, and his method worked, he was awarded the marks....
Tony.
For those who are interpreting speed as the rotational speed of the wheel, and the belt matches this rotational speed (though as maxro points out with his gear analogy the belt speed can't actually be equal, but if you convert from angular velocity to linear, you can get the equivalent speed) then if the belt matches this speed then the plane would have to be remaining stationary from the refference point of the ground and would attain no airspeed.... this is of course not physically possible due to the propulsion mechanism. but never the less the question in no way mentions that standard physics applies.
SO this is why IMO we have some saying it will take off and some saying it wont. IT all comes down to how you interpret SPEED and relative motion, not whether you know how a jet engine works or not 😉
yes the speed of the wheels in reality has nothing to do with whether the plane takes off or not, but within the context of the question depending on how you interpret it, it has everything to do with it!!
IT is an extremely poor question.
I have an uncle who at uni got the question
you are on the roof of a building and have a theotelite how would you determine the height of the building?
he said he would drop the theotelite off the building and time its decent and based on gravity being 9.8M/s/s he would calculate the height of the building. He was marked wrong (probably for being a smartass) but when he protested that the question didn't stipulate how the theotelite was to be used, and his method worked, he was awarded the marks....
Tony.
anatech said:
so the belt rolls along the ground? I think not.
I think that like any real conveyer belt, it rotates around axles that are fixed in relation to the earth. Yes, the surface of the belt moves, but its intire self does not translate along.
I agree with Wintermute and RDFan on this one. The question is poorly stated and too open to (mis-) interpretation. Further, it involves a completely impossible situation.
Max
Hi Max,
Agreed, the wording of the question is what causes much confusion.
But the original question infers that the belt is not the limiting factor in motion in as much as it's as big as the runway.
So as far as the question is concerned, the surface of the belt is not fixed to the ground in any way. Also the belt opposes the rotation of the wheel but adds no additional drag.
The surface does.
-Chris
Agreed, the wording of the question is what causes much confusion.
But the original question infers that the belt is not the limiting factor in motion in as much as it's as big as the runway.
So as far as the question is concerned, the surface of the belt is not fixed to the ground in any way. Also the belt opposes the rotation of the wheel but adds no additional drag.
The surface does.
-Chris
anatech said:
This is what I thought at first. However, notice the part about the belt exactly matching the speed of the wheels. No matter how fast the wheels spin, the belt sipns backwards at the SAME speed. Therefore the wheels' axles and thus the plane can make no progress relative to the air/earth.
Essentially the plane is constantly being tripped-up by the conveyer belt.
It is a completely impossible scenario in which all the force from the propeller must be absorbed by the wheel bearings, tires and belt.
Max
anatech said:
yes the belt moves, but it (like the wheel) must be decoupled from the suroundings by bearings, so the structure around which the belt moves is stationary with relation to the earth. Unless you are thinking along the lines of a tank track???? where the bottom side of the belt is in contact with the ground and is therefore moving backward* at the same rate as the forward movement of the plane??? now that's an interpretation I hadn't thought of!!!
Tony.
*edited to correct moving forward to backwards.
the plane gets its movements from the jets... so the converbelt could not match the speed of the wheels because the plane would be moving itself through the air. so yes. the plane would take off, but the conveyor cannot match the speed of the wheels.
Hi Max,
Okay, what do the wheels do if the belt moves exactly opposite to their motion? My guess is the wheels do not rotate as the plane moves along. But even if they do, then it's twice as fast. Either way, this does not affect the thrust acting on the air that moves the airplane forward.
The wheels are decoupled from the aircraft by their bearings.
The belt is decoupled from the earth from it's bearings.
Plane flies.
-Chris
Okay, what do the wheels do if the belt moves exactly opposite to their motion? My guess is the wheels do not rotate as the plane moves along. But even if they do, then it's twice as fast. Either way, this does not affect the thrust acting on the air that moves the airplane forward.
The wheels are decoupled from the aircraft by their bearings.
The belt is decoupled from the earth from it's bearings.
Plane flies.
-Chris
escalade182 said:
Hah, you violated the belt/wheel speed matching stipulation of the scenario.
Max
Hi Tony,
The belt moves freely to oppose the motion of the wheels. However the belt is as big as the runway. The belt is fixed at each end by bearings that allow it to move. It has it's own power source.
Therefore, if you paint a spot below the airplane on the belt, the spot is free to move in relation to the earth.
The wheel bearings allow the wheels to move freely with minimal resistance to the motion of the aircraft. Doesn't matter which way you spin them.
When you force the body of the aircraft forward, it's going to move. It doesn't care whether the tires are rotating or not as long as the gear supports can move. This they can do as the bearings allow rotation if required.
-Chris
The belt moves freely to oppose the motion of the wheels. However the belt is as big as the runway. The belt is fixed at each end by bearings that allow it to move. It has it's own power source.
Therefore, if you paint a spot below the airplane on the belt, the spot is free to move in relation to the earth.
The wheel bearings allow the wheels to move freely with minimal resistance to the motion of the aircraft. Doesn't matter which way you spin them.
When you force the body of the aircraft forward, it's going to move. It doesn't care whether the tires are rotating or not as long as the gear supports can move. This they can do as the bearings allow rotation if required.
-Chris
OK lets substitute car with wheels providing propultion for plane (yes I know this is completely different, but humour me 😉 )
Provided the speed the coveyor is moving backwards matches the forward speed that would be obtained by the wheels driving the car the car would remain stationary with regard to an observer not on the belt true???
now if you only think about the wheels on the plane in the same respect as the car example above ignoring that the jet isn't powering the wheels, just think about their speed relative to the belt, then you can see the conundrum 🙂 there can be no forward movement relative to an observation point outside of the belt if the speeds are matched (depending once again on your interpretation of speed). This of course as I said before is just one interpretation of the question 🙂
Tony.
Provided the speed the coveyor is moving backwards matches the forward speed that would be obtained by the wheels driving the car the car would remain stationary with regard to an observer not on the belt true???
now if you only think about the wheels on the plane in the same respect as the car example above ignoring that the jet isn't powering the wheels, just think about their speed relative to the belt, then you can see the conundrum 🙂 there can be no forward movement relative to an observation point outside of the belt if the speeds are matched (depending once again on your interpretation of speed). This of course as I said before is just one interpretation of the question 🙂
Tony.
Hi Tony,
I agree completely with you on the car thing. That's the same as an exercise walker except you regulate your speed to match.
Notice that the car needs to rotate the wheels to generate motion whereas the wheels on the plane move in response to motion. Completely different kettle of fish.
The point of reference in the plane is the air and plane, not the conveyer surface. The conveyer surface has no reference to the earth either, except it can't jump up in the air.
Since the wheels can rotate freely on the plane, the conveyer can spin them where ever it wants. That will have minimal effect on the plane responding to the thrust of the engines.
Now, if the pilot jumped on the brakes, the plane just became coupled to the wheels and conveyer belt. At this point, the plane can not take off. But then it couldn't if on the tarmac either.
-Chris
I agree completely with you on the car thing. That's the same as an exercise walker except you regulate your speed to match.
Notice that the car needs to rotate the wheels to generate motion whereas the wheels on the plane move in response to motion. Completely different kettle of fish.
The point of reference in the plane is the air and plane, not the conveyer surface. The conveyer surface has no reference to the earth either, except it can't jump up in the air.
Since the wheels can rotate freely on the plane, the conveyer can spin them where ever it wants. That will have minimal effect on the plane responding to the thrust of the engines.
Now, if the pilot jumped on the brakes, the plane just became coupled to the wheels and conveyer belt. At this point, the plane can not take off. But then it couldn't if on the tarmac either.
-Chris
anatech said:
It doesn't matter how fast or slow the wheels spin. They are still attached by their axles to the plane.
And, the belt is still attached to the earth by its axles.
Now, the surface speed of the wheels must match the surface speed if the belt. That much is given.
So, assuming the belt does not roll along like a tank track, its axles are fixed to the earth.
The only way for the plane's wheel axles to make progress relative to the air/earth is for them to move in relation to the belt's axles. And for this to happen, the wheels' must spin faster than the belt.
Rule broken. Can't happen. No plane movement. No lift. No take off.
I don't know what happens to the thrust from the plane's engine. It can't move the plane forward, or cause it to smash its nose into the ground trying to take off. Both of these motions break the belt/wheel speed matching stipulation.
Max
Hi Chris,
I agree with you 🙂 except that depending on the interpretation of the statement that the conveyor matches the speed of the wheels, this can either make or break the question 🙂
I'm happy that in reality the plane would take off!
What I think is that there are certain interpreations of the question, which while not physically possible violate the requirement that the speeds be matched. Maybe I'm being a bit pedantic 🙂
I guess what it comes down to, is whether the jet engine is a red herring of whether the wheels are 😉 ie whether the question is about the propultion mechanism and is purely physics based, or whether the question is about relative motion and physics (at least the propultion side of it) has been thrown out the window!
Unfortuanately without knowing what the intent of the original question was, I don't think it is possible to reach a consensus 🙂
Tony.
I agree with you 🙂 except that depending on the interpretation of the statement that the conveyor matches the speed of the wheels, this can either make or break the question 🙂
I'm happy that in reality the plane would take off!
What I think is that there are certain interpreations of the question, which while not physically possible violate the requirement that the speeds be matched. Maybe I'm being a bit pedantic 🙂
I guess what it comes down to, is whether the jet engine is a red herring of whether the wheels are 😉 ie whether the question is about the propultion mechanism and is purely physics based, or whether the question is about relative motion and physics (at least the propultion side of it) has been thrown out the window!
Unfortuanately without knowing what the intent of the original question was, I don't think it is possible to reach a consensus 🙂
Tony.
Hi Max,
Well, we have two possibilties. The wheels move or they don't. If they move, how fast? Doesn't matter. The wheels are designed to rotate freely on their axials. Plane takes off.
See post above. The pilot needs to apply the brakes to couple the motion of the plane to the belt.
-Chris
Well, we have two possibilties. The wheels move or they don't. If they move, how fast? Doesn't matter. The wheels are designed to rotate freely on their axials. Plane takes off.
See post above. The pilot needs to apply the brakes to couple the motion of the plane to the belt.
-Chris
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