Physics Puzzle

[johnny_t]

It bloody well is !!

If we were talking about the point at the centre of the wheels, and for every inch that moved forward the belt moved it an inch back, then it would not go anywhere. However, when talking about matching the speed the circumference of the wheels are going round at, then the plane can still move forward happily. Honest.

Here are two scenarios to imagine. Firstly the plane is sat on the runway with a cable going to a winch directly in front of it. Fire up the runway and the plane stays still. Increase the runway speed to as fast as you like and the plane still stays where it is, with no discernable increase in the tension on the cable, as the wheels are spinning beneath it freely. Turn the winch and the plane moves forward, even if you cranked the runway speed to compensate 3 x for the wheel speed.


Alternatively, imagine you are sat in a little trolley, on a conveyor belt, facing down a vertical slope (or make it 89 degrees if you are worried about falling off) - No matter how fast the conveyor goes upwards, nothing is going to stop you rolling downwards. Change it to an ice-conveyor and change the wheels to skates - same thing applies. Don't be fooled into thinking they have something to do with drive in either direction just because they are wheels.


In the first scenario the winch is equating to the thrust of the plane, in the second its gravity. Either way the plane is going to move forward and take off (and, lets face it, no-one would have bothered asking the question if it wouldn't).

 

[joe-90]

Very good Johnny, if there is anyone that can't grasp that - well they must be cerebrally challenged.

 

[johnny_t]

erm......thanks mate !!



(checks and rechecks carefully for hidden sarcasm..... )

 

[hermes]

It b****y well is !!

If we were talking about the point at the centre of the wheels, and for every inch that moved forward the belt moved it an inch back, then it would not go anywhere. However, when talking about matching the speed the circumference of the wheels are going round at, then the plane can still move forward happily. Honest.

Here are two scenarios to imagine. Firstly the plane is sat on the runway with a cable going to a winch directly in front of it. Fire up the runway and the plane stays still. Increase the runway speed to as fast as you like and the plane still stays where it is, with no discernable increase in the tension on the cable, as the wheels are spinning beneath it freely. Turn the winch and the plane moves forward, even if you cranked the runway speed to compensate 3 x for the wheel speed.


Alternatively, imagine you are sat in a little trolley, on a conveyor belt, facing down a vertical slope (or make it 89 degrees if you are worried about falling off) - No matter how fast the conveyor goes upwards, nothing is going to stop you rolling downwards. Change it to an ice-conveyor and change the wheels to skates - same thing applies. Don't be fooled into thinking they have something to do with drive in either direction just because they are wheels.


In the first scenario the winch is equating to the thrust of the plane, in the second its gravity. Either way the plane is going to move forward and take off (and, lets face it, no-one would have bothered asking the question if it wouldn't).

Yes, I see what you mean. My point though is this:

If the speed of the belt is the same as the speed of the wheels, then surely we are talking about their respective speeds relative to the ground.
Therefore, whether you are talking about the speed of the centre of the wheel or the circumferential speed of the wheel, if the conveyor belt is matching that speed, then it seems to me that relative to the ground, the plane must be stationary.
I like your winch analogy but if the plane is moving towards the winch then the wheels of the plane must be moving faster than the conveyor, unless the winch is moving relative to the ground. It doesn't matter if you crank up the belt speed, whether the belt is moving at 1 m/s or 1000mph, if you pull the plane towards to winch then the speed of the wheels will be exceeding that of the belt.

PS Adam no I wouldn't stand in front of the belt, I'm not b****y stupid enough to test my own theories in that way!

 

[empip]

The engines are thrusting against the air,...

That doesn't in itself move the aircraft.
Tis the reaction to that thrust acting upon the apparatus, causing the movement.
Shove a little volume at high speed pure jet, or a greater volume at lower speed ducted bypass jet...

If I put a gas turbine in outer space and fed it with about 1.5 tons a second of air, besides it's other requirements it would jolly along more or less quite nicely...

A rocket doesn't have anything to thrust against except itself, it unfortunately has to carry all it's requirements for propulsion.

 

[joe-90]

As i said on the first page - it's all down to Newton's Law of motion (3rd).


joe

 

[Nige F]

Nah.it`s Paddy Law`s motion (turd)

 

[Zampa]

Isnt it about Chilluk raised his or her head and settled this?

 

[empip]

Perhaps we need to think about the amount of thrust a jet engine may provide.
Very rough figures.
A Boeing 747 weighs 545,000 lbs (empty !!) max take off wt 870,000 lbs
Each of 4 engines have 57,000 lbs of thrust total available 228,000 lbs.
That's thrust to wt ratio of 0.26
We know plenty about locomotives and road vehicles with respect to tractive resistance.
It is said that a pneumatic tyred vehicle on level, smooth tarmac has tractive resisitance of around 25-35 lb per ton this means it would require about 35lb 'push' for every 1 ton weight to maintain a steady speed. ( pushing the car? A big shove to overcome inertia or static friction, then it gets easier - on level road - to maintain steady pace)
So our 228,000 lbs of 'push' should be ok to roll 6,500 tons at a steady pace... Or to send a mere 380 ton 747 accelerating down a runway, a ratio of about 17:1 .. The excess thrust takes care of all the components involved in 'drag' or holding the aircraft back.. and enables the necessary acceleration to take off speed over the prescribed runway distance.

If the drag equals the thrust, then there will be no further acceleration.

That factor for an F16 fighter at max take off weight is a massive 42:1.. With thrust to wt ratio of 0.89 .. I guess that means she could not actually sustain a vertical climb, that would require a ratio greater than 1 - not bad for 19 tons of equipment ! .. becomes some performer when the ordnance has been dropped and some fuel burnt off ....

 

[AndersonC]

now all he needs is a con belt a few miles long,

The belt only needs to be 150 metres long.

Even if you mean 300m long, i.e. 150m between the end rollers, I'm not sure any of his planes can take off in 150m....

A Eurofighter on full reheat with a low weapons and fuel payload can do it.......

And the thrust:weight ratio on this aircraft does allow it to climb vertically also.

 

[kendor]

No - completely wrong.

The engines are thrusting against the air, not via the wheels to the runway. It doesn't matter whether the runway is static, or moves forwards or moves backwards relative to the wheels in order to make the wheels rotate normally/slower/faster respectively, the plane will still move through the air as normal.

what if the plane had it's brakes on seriously though in order to move forward the plane will have to travel the circumference of it's wheels but the runway is matching any advancement in distance of the wheel ie the runway cancels out the distance of the circumference of the wheel every complete rotation therefore the plane is trying to thrust itself forward but is merely skidding if that's the right word to use and because it isn't gaining distance it isn't moving and therefore no airflow over the wings for lift. even if the pilot carried on applying thrust the the wheels would spin faster but gain no ground.
I daresay there will be a point when the plane veers off and crashes though. unless engineered to withstand infinite thrust and wheel speed.

 

[Zampa]

Perhaps we need to think about the amount of thrust a jet engine may provide.
Very rough figures.
A Boeing 747 weighs 545,000 lbs (empty !!) max take off wt 870,000 lbs
Each of 4 engines have 57,000 lbs of thrust total available 228,000 lbs.
That's thrust to wt ratio of 0.26
We know plenty about locomotives and road vehicles with respect to tractive resistance.
It is said that a pneumatic tyred vehicle on level, smooth tarmac has tractive resisitance of around 25-35 lb per ton this means it would require about 35lb 'push' for every 1 ton weight to maintain a steady speed. ( pushing the car? A big shove to overcome inertia or static friction, then it gets easier - on level road - to maintain steady pace)
So our 228,000 lbs of 'push' should be ok to roll 6,500 tons at a steady pace... Or to send a mere 380 ton 747 accelerating down a runway, a ratio of about 17:1 .. The excess thrust takes care of all the components involved in 'drag' or holding the aircraft back.. and enables the necessary acceleration to take off speed over the prescribed runway distance.

If the drag equals the thrust, then there will be no further acceleration.

That factor for an F16 fighter at max take off weight is a massive 42:1.. With thrust to wt ratio of 0.89 .. I guess that means she could not actually sustain a vertical climb, that would require a ratio greater than 1 - not bad for 19 tons of equipment ! .. becomes some performer when the ordnance has been dropped and some fuel burnt off ....

So what are you saying then?

 

[empip]

http://www.avweb.com/news/columns/191034-1.html

Enjoy.

 

[BOB.DOLE]

so i vas right all along just as usual

 

[ban-all-sheds]

Explain why you can no longer walk.

Because if you could walk, the belt would no longer be keeping up with the wheels.

So you're saying that as soon as the conveyor belt starts up, you would no longer be able to walk along, pushing the small cart with the high quality bearings in the wheels?

Why is this?