Physics Puzzle

[johnny_t]

Do you know, I've read and re-read this, but still don't get it. Why is the plane's situation any different from, say, a car on a rolling road - That doesn't go anywhere with its wheels spinning twice as fast as usual.

 

[JohnD]

Because the plane doesn't put any power through its wheels; the wheels are irrelevant to its forward motion which is powered by the propellors or jets.

 

[johnny_t]

Yeah but....

If the plane was on solid ground, the wheels would still turn as it moved forward, by a distance around their circumference equal to the distance the plane has moved forward, wouldn't it ?

So the rolling runway is then moving it backwards by the same amount, doesn't it ? So doesn't it stand still ?

I don't doubt you are right, I just can't get it.

 

[JohnD]

I think, if I've got it, the jets push the plane forward by a metre.

As this is happening, the wheels have to turn enough to keep up.

The wheels would turn by 1 metre of circumference. But this makes the runway move one metre iin the opposite direction. So, the wheels find they have moved two metres along the moving runway to accomodate 1 metre's movement of the plane.

 

[markie]

It would take off because there is more friction at the wheels on the con belt, than the plane against the air, so it would need a hell of a long con belt to take off.

PS If E.T.'s mate who was peddling his bike, was on a treadmill would he be able to fly, like he did in the movie, ?

 

[2scoops0406]

So the answer is yes, the plane will take off.
If the conveyor belt moves in the opposite direction to the wheels (at equal but opposite speed) the only result is the wheels will spin twice as fast as they would normally. The aeroplane will still move, the lift of the aeroplane is given by the speed of the air over the top of the wing being faster than the speed of air under the wing resulting in a depression above the wing and hence lift.

But as the original statement says, the conveyor adjusts it's speed continually to match that of the wheels, therefore no it won't take off.

As there is no wind, the ground speed will be zero, and so will the air speed.

Also how does the speed air change over the top of the wing of an aeroplane, I'd be interested to know the answer to that one !! loaded question BTW.

 

[joe-90]

You are all comparing two totally independent unrelated systems. It matters not what the wheels are doing. When thrust is applied to the engines the jet will move forward. The bulk of the jet is moved by the engines. The wheels would always move at X + forward momentum. The movement of the jet cannot be compensted by rotating wheels underneath. If the conveyor belt really existed in the real world it would cause drag due to friction as the speed increased and the jet would take off when it's air velocity over the wings reached its designed take-off speed.



joe

 

[2scoops0406]

The wheels would always move at X + forward momentum.joe

What has the [unstated] mass of the areoplane got to do with it

 

[joe-90]

The wheels would always move at X + forward momentum.joe

What has the [unstated] mass of the areoplane got to do with it

It is pushed forward by thrust. Newton's third law. (Equal and opposite reaction)

 

[Spark123]

But as the original statement says, the conveyor adjusts it's speed continually to match that of the wheels, therefore no it won't take off.

As there is no wind, the ground speed will be zero, and so will the air speed.

Also how does the speed air change over the top of the wing of an aeroplane, I'd be interested to know the answer to that one !! loaded question BTW.

It depends on how you perceive the speed of the wheel, if you are standing still next to the runway (i.e. on terra firma) and measuring the speed to a position on the wheel it holds true, the wheel will have to spin at twice the rate it would have to cover the same ground. If you try to work it out by the speed of the revolving wheel it becomes confusing as it is not possible to have the runway moving as fast as a wheel on it in the opposite direction, the only time this holds true is when the aircraft is stationary.

 

[2scoops0406]

indeed but as we don't know the coefficient of friction of the tyres, we have to assume by the statement that the conveyor matches the speed of the tyres that it must apply an equal and opposite force, therefore no forward motion. Lots of heat yes, but lift no.

 

[Spark123]

Some info on aerofoils here:
http://en.wikipedia.org/wiki/Airfoil

 

[joe-90]

Sorry guys but you are arguing from two points. It's like saying "I can lift 100 Kilos so if I grab my angles and lift I'll levitate."

The wheels have nothing to do with the thrust and the mass of the aircraft.
Go off and read what Newton has told you. Go off and think about what the great joe-90 has told you. The conveyor would be speed X. The speed will always be T(thrust)+x. The jet is driven by the engines - not the wheels. Combining the two is like picking yourself up - unrelated.



joe

 

[2scoops0406]

Some info on aerofoils here:
http://en.wikipedia.org/wiki/Airfoil[/QUOTE]

I did say it was a loaded question, I don't think I necessarily believe that fliud dynamics (non compressible system) correlate to a gaseous system (compressible), this is how planes are believed to fly (Bernoulli’s principle).

You will have to hunt high and low for a good explanation, 'cos even the NASA websites hasn't convinced me yet.

It's a very complicated and hotly contested subject, and one I am willingly able to say that I don't understand, but nevertheless interesting.

 

[joe-90]

Let me explain:

Imagine I have a roller skate in my hand and I sit it on a conveyor belt. Now if I move that roller skate by applying thrust (through my arm) and move the roller skate forward, it matters not what the conveyor belt is doing, it cannot stop me moving the roller skate forward - and it's the same with the jet. It matters not what the conveyor belt is doing, the Law of science applies to the 'equal and opposit reaction of the thrust'. Just as my arm would push the roller skate forward - so will the engines push the jet forward to take-off velocity.

Geddit?


joe