Physics Puzzle

[doitall]

No it doesn't

Take a magical run way Bristol Lulsgate runway 27

I'm sat in me plane doing 100mph, and the runway is doing 100mph the opposite direction, would I break the starting tape or would the plane be stalled.

I dont know the take off speed of your plane so couldnt say if it would take off or not but if it was 100mph then youd be airborne

85knots but that is irrelevant, read the question and answer.

 

[tim west]

if its going fast enough then itll take off, the moving runway is irrelevant

 

[doitall]

As I said if You run up an escalator at 25mph, that is going 25 mph going down would you get to the top

 

[doitall]

if its going fast enough then itll take off, the moving runway is irrelevant

No, the runway is matching my speed so the plane is stalled, to get take off I need to be rolling forward at 85knots

 

[tim west]

As I said if You run up an escalator at 25mph, that is going 25 mph going down would you get to the top

Thats a totally different argument we are talking of a plane on a runway that has wheels

take for example you run up the conveyor to maximum speed, the plane wouldnt move its wheels would spin very fast but the plane would be static

the runway doesnt affect the speed of the plane

now apply enough thrust and the plane will move through the air and take off.

 

[doitall]

You still don't get it.

The airframe need forward motion to get lift.

If the runway/conveyor is matching that forward motion then the plane is stalled, it doesn't matter how fast the wheels are turning, because the surface is moving in the opposite direction at the same speed.

 

[noseall]

the wheels and the runway can fight each other all day long.

the runway and wheels can go in opposite directions till doomsday.

however, as soon as you power up the engines, air and its density come into play.

the engines will react with the air and push the aircraft forwards, regardless of the fight below.

as John D says: 'tis all hypothetical twaddle in any case. it relies on the wheels being free of friction. this friction free scenario means that the wheels and runway do what ever they want allowing the engines to react with the AIR.

 

[tim west]

You still don't get it.

The airframe need forward motion to get lift.

If the runway/conveyor is matching that forward motion then the plane is stalled, it doesn't matter how fast the wheels are turning, because the surface is moving in the opposite direction at the same speed.

I get it, im afraid it's you that doesnt, the bearings are frictionless therefore they are basically detached from the airframes axle,
the inner surface of the wheel and the outer suface of the axle are free from each other the wheel applies no friction (theoretically remember) to the surface of the axle and therefore free spins as fast as it is driven by its traction via the tyres to the runway.

the axle centerpoint remains static in space as there is nothing pushing it backwards

apply thrust and the planes airframe moves in the opposite direction to the thrust (newtons law) enough thrust= takeoff speed.

 

[sooey]

You still don't get it.

The airframe need forward motion to get lift.

If the runway/conveyor is matching that forward motion then the plane is stalled, it doesn't matter how fast the wheels are turning, because the surface is moving in the opposite direction at the same speed.[/quote

You're thinking that the wheels are providing the forward motion. They're not, the engines are, the wheels are just spinning freely on their axles. The faster the conveyor moves the faster the wheels will spin, but they won't stop the plane from moving forwards in reaction to the thrust of the engines.

 

[noseall]

if the theory incorporated some kind of air speed management that countered the thrust of the engines then this would have an effect.

otherwise - take off.

 

[tim west]

if the theory incorporated some kind of air speed management that countered the thrust of the engines then this would have an effect.

otherwise - take off.

very true , the runway would still be irrelevant though

 

[noseall]

just to discount the moving walkway theory......

ok the guy is running and matching the speed of the walkway by tread-milling along, but imagine he has a jet pack strapped to his back and fires it up......


the jet pack reacts with the air and NOT with the ground.

geddit.

 

[doitall]

You're thinking that the wheels are providing the forward motion. They're not, the engines are, the wheels are just spinning freely on their axles. The faster the conveyor moves the faster the wheels will spin, but they won't stop the plane from moving forwards in reaction to the thrust of the engines.

It doesn't matter what the forward propulsion is or how much.

for the plane to roll forward the wheels have to turn, if the surface is matching that in the opposite direction, then it cannot travel forward

 

[tim west]

just to discount the moving walkway theory......

ok the guy is running and matching the speed of the walkway by tread-milling along, but imagine he has a jet pack strapped to his back and fires it up......


the jet pack reacts with the air and NOT with the ground.

geddit.

I know its the case with galloping horses, not sure with humans but maybe also that every stride the runner would actually leave the ground and be airborne?

Thinking about it yes you do leave the ground with each stride.

 

[sooey]

just to discount the moving walkway theory......

ok the guy is running and matching the speed of the walkway by tread-milling along, but imagine he has a jet pack strapped to his back and fires it up......


the jet pack reacts with the air and NOT with the ground.

geddit.

Nothing to do with the air (there is none in space where rockets can move)
Every action has an equal and opposite reaction.