A plane on a treadmill - physics question

[Notarealsignguy]

Oh, gotcha! I included the lack of friction because I thought it was funny that it was brought up earlier.

Also....what's a people mover? I haven't left my island in two years so I'm technically still in 2018.

like a flat escalator... or a really long treadmill that you stand on and it carries you down the concourse. Although, my analogy was flawed as you walk in the same direction that it is moving. If you put your bag on it and walked the opposite direction next to it, you could pull it at whatever speed you were walking.

 

[Taryn]

Oh nice! Now I really have get off the island and check it out.

 

[Andy D]

It is essentially acting like a hovercraft

It's not a hovercraft, it's a wheel, with its own laws of physics... forget the plane and everything else, just tell
me one thing: how a wheel can move forward if it's turning at the same speed as the treadmill?

 

[Notarealsignguy]

It's not a hovercraft, it's a wheel, with its own laws of physics... forget the plane and everything else, just tell
me one thing: how a wheel can move forward if it's turning at the same speed as the treadmill?

A wheel can spin and not move.

 

[Andy D]

A wheel can spin and not move.

Correct, but a wheel can't move unless it's spinning at a faster speed than the ground..

 

[signage]

If you had a tread mill big enough to do that it would move alot of air which in turns effects the wings, so unless you had a fan to cancel the air being moved by the tread mill, if the air movement was great enough the plane would lift due to the unequal air pressure and would now be flying? Correct?

 

[Notarealsignguy]

Correct, but a wheel can't move unless it's spinning at a faster speed than the ground..

Not true at all. A spare tire on a car does not spin but moves as fast as what it is connected to. A car doing a burnout has the tires spinning but stays in place. Same with a car on ice. To move it requires a connection which could be physical or friction.
Chances are good that even if the plane had its brakes applied, the thrust would overcome the frictional bond of the treadmill and either move forward or move backward at a slower speed than the treadmill is moving.

 

[Dan360]

A stationary wheel can move with enough force. But that's not really the problem here, the treadmill would never produce enough backwards force on the plane to negate the engines.

 

[Gino]

You're looking at this all wrong. Instead of a treadmill, we need a motorized buncha wheels/tires ON the landing strip. With their spinning capability, the plane could take off and land without wheels in the first place. Less fuel to power it up, less braking power as the wheels will brake on their own and cancel out all that fuel..... the payload'll be much lighter giving it more distance and room inside..

 

[Taryn]

You're looking at this all wrong. Instead of a treadmill, we need a motorized buncha wheels/tires ON the landing strip. With their spinning capability, the plane could take off and land without wheels in the first place. Less fuel to power it up, less braking power as the wheels will brake on their own and cancel out all that fuel..... the payload'll be much lighter giving it more distance and room inside..

I think you're onto something big here!

 

[Taryn]

Yes, except if there were no friction present (which is my comparison to the friction real world wheels have vs thrust generated) the plane itself would sit still as there is no outside force acting on it. First law, object at rest tends to stay at rest unless acted upon by an outside force. The outside force here is our treadmill. Treadmill cannot do anything to the plane since the friction on the wheels is negligible.

Why would it stay still, though? The plane itself is generating a force to push it forward (the jets), and because every action has to have an equal and opposite reaction, that is usually enough to start pushing it forward. In this case, the plane does not need an outside force to start going.

 

[Notarealsignguy]

With the engines on, the wheels would not be spinning at the same rate as they would with the plane on solid ground at a given speed. They would be spinning faster than the rate of the treadmill since the engines would be propelling it forward and gaining distance would would require more revolutions of the tire. The RPMs do not equate the speed the same way when the ground is a variable. Same would be true in reverse. To answer your question with a question, how could the plane go the speed of the treadmill without the wheels even turning?
I feel like this is a riddle and keep waiting for the clever response.

 

[Andy D]

Nothing reacts instantly, but let's assume
the treadmill matches speed almost instantly, like in a millisecond (1/1000 of a second) and the plane moved forward at 5mph:
1st millisecond plane 5mph -treadmill matches.
2nd millisecond plane's tire is forced to 10mph -treadmill matches.
3rd millisecond plane's tire is forced to 15mph -treadmill matches.
4th millisecond plane's tire is forced to 20mph -treadmill matches.
After a full second the wheels are spinning at 5,000mph.
Two forces that are plied against each other, unconstrained, compounding each other instantly, would become incredibly volatile within seconds.

 

[Texas_Signmaker]

Nothing reacts instantly, but let's assume
the treadmill matches speed almost instantly, like in a millisecond (1/1000 of a second) and the plane moved forward at 5mph:
1st millisecond plane 5mph -treadmill matches.
2nd millisecond plane's tire is forced to 10mph -treadmill matches.
3rd millisecond plane's tire is forced to 15mph -treadmill matches.
4th millisecond plane's tire is forced to 20mph -treadmill matches.
After a full second the wheels are spinning at 5,000mph.
Two forces that are plied against each other, unconstrained, compounding each other instantly, would become incredibly volatile within seconds.

I'm coming down to the Burg next week... I was going to fly down there but it will probably be faster to just drive the 8 hours in my truck instead of sitting and spinning on those runways.

 

[Andy D]

I'm coming down to the Burg next week... I was going to fly down there but it will probably be faster to just drive the 8 hours in my truck instead of sitting and spinning on those runways.

LOL, if you do, come by the shop...btw did you end up getting that channel letter job I told you about?

 

[Texas_Signmaker]

LOL, if you do, come by the shop...btw did you end up getting that channel letter job I told you about?

No, they wanted to replace their sign with a bigger one but didnt really care for the "bigger sign" price... but that's most people anyway.

 

[Andy D]

I
But it all boils down to whether or not the helium filled balloon inside the plane will move forward or backwards when the plane takes off!

Because air is like water in almost every regard except weight/density, it would act in the same way as if a submarine full of water, accelerated quickly, the water inside would be forced to the back of the sub and compressed, a water balloon would move towards the back of the sub slightly until the sub met the apex of its speed. The water would then decompress, pushing the water balloon forwards, close to its original position.
Edited to say: I just noticed you said helium balloon, in which case the compression of the air would force the balloon forwards.

 

[bob]

There is no confusion about what propels the plane; it's the thrust of the engines, not the wheels, the engine's thrust is independent from the ground & the wheels are free moving, not motorized...That is understood, Okay?
But the wheels are just as critical to flight as the wings and engines... answer this one question:
A plane must be moving forward at around 180mph to take off, how will a plane move forward if it's wheels cannot go faster than the ground?

Try to follow along, as long as the thrust generated by the airplane can overcome the friction of the wheels the airplane will move forward. When taking off an airplane's wheels serve only to hold it off the ground, they are not a factor in taking off. It makes no difference how fast and in what direction they might be turning.

 

[Johnny Best]

 

[Texas_Signmaker]

Sounds like the Bernie Sander's plane on the DNC's runway... They won't let that thing take off.