Plane

[RichA]

I'm going to have to say that this scenario has swayed well off track, I struggle to see the comparison. Surveyors with measuring wheels, thrusting those measuring wheels forwards by exerting a force directly onto belt. Compared to a plane on a conveyor belt, being thrust forwards by a force exerted in the air.

As for friction. Sure, it won't be zero, but it will be negligible. Especially if you are comfortable with the wheels being considered "free-wheeling"

You're missing the fact that forward movement requires breaking through the friction of the wheels' traction on the belt, as they have to go at the same speed.
Sure the jet engines can accomplish this but even reinforced plane tyres are not designed to withstand the level of friction that gets the plane to takeoff speed.

 

[Swango1980]

I'm not saying that you can't move in that scenario. I'm saying that as soon as somebody does push you forwards the wheels on your roller skates are rotating faster than the belt, which the fictional scenario does not allow.
That, my friend, is pretty simple maths.

We have agreed on this point for the last 3/4 days though. We have said that if a plane tries to take off, and the belt is programmed to move backwards, then the wheel speed and belt speed cannot match. We are not arguing against that point. In that scenario, the belt would also not be counteracting rotation.

So, to me, there are three main answers if you agree a belt cannot stop a jet engine from propelling a plane forwards AND your interpretation is that the belt MUST move the same speed backwards as the wheels are moving forwards

1. That it is impossible. So, the question cannot be answered. There is no point in saying the scenario is impossible to achieve and then giving a definitive answer as to what the plane actually does.
2. That is could be programmed by a programmer, but the programmer would be unable to get the belt to stop the plane taking off regardless
3. It is a special plane that is thrust forward by driving the wheels directly.

 

[RichA]

Everything you are saying is fiction.

Deep breath.
You're on the belt on your roller skates. Your mate is holding you stationary - your wheels are travelling at the same speed as the belt.
Your mate pushes you forward at 3mph. This can only be achieved if your wheels are travelling 3mph faster than the belt.
If the wheels and belt's speeds are identical then your wheels are slipping on the belt. There cannot be perfect traction.

 

[Swango1980]

You're missing the fact that forward movement requires breaking through the friction of the wheels' traction on the belt, as they have to go at the same speed.
Sure the jet engines can accomplish this but even reinforced plane tyres are not designed to withstand the level of friction that gets the plane to takeoff speed.

Nope, I'm not missing that fact. Read my post #711.

 

[Deleted member 35927]

Deep breath.
You're on the belt on your roller skates. Your mate is holding you stationary - your wheels are travelling at the same speed as the belt.
Your mate pushes you forward at 3mph. This can only be achieved if your wheels are travelling 3mph faster than the belt.
If the wheels and belt's speeds are identical then your wheels are slipping on the belt. There cannot be perfect traction.

I truly don't understand what your point is anymore.

 

[RichA]

I truly don't understand what your point is anymore.

The speed of the wheels and the speed of the belt are always identical.
The plane itself requires forward velocity of 180mph to take off.
So in order to take off the plane's forward velocity must be 180mph greater than the speed of its wheels and the belt.
So at take off the wheels are rubbing backwards on the conveyor belt at 180mph.
Massive friction. Mechanical failure.

 

[Swango1980]

The speed of the wheels and the speed of the belt are always identical.
The plane itself requires forward velocity of 180mph to take off.
So in order to take off the plane's forward velocity must be 180mph greater than the speed of its wheels and the belt.
So at take off the wheels are rubbing backwards on the conveyor belt at 180mph.
Massive friction. Mechanical failure.

True. So, plane either takes off, or crashes depending on the damage to the wheels.

Unless, of course, the surface of the conveyor belt is made of a very slippy surface similar to ice. The wheels could then skid without mechanical failure

 

[RichA]

True. So, plane either takes off, or crashes depending on the damage to the wheels.

Unless, of course, the surface of the conveyor belt is made of a very slippy surface similar to ice. The wheels could then skid without mechanical failure

Yep. In the scenario where there is perfectly zero friction between tyres and conveyor belt, the wheels don't actually turn at all and the plane happily slides down the stationary belt to takeoff.

Assuming the friction of the wheel bearings is every so slightly more than perfectly zero.

 

[Deleted member 35927]

Yep. In the scenario where there is perfectly zero friction between tyres and conveyor belt, the wheels don't actually turn at all and the plane happily slides down the stationary belt to takeoff.

Assuming the friction of the wheel bearings is every so slightly more than perfectly zero.

Plane wheels rotate freely in either direction, where are you getting all this friction?

 

[RichA]

Plane wheels rotate freely in either direction, where are you getting all this friction?

How are you missing it?

 

[Deleted member 35927]

How are you missing it?

You post so much imaginary stuff it's tricky to know what you are talking about tbh

 

[RichA]

You post so much imaginary stuff it's tricky to know what you are talking about tbh

A wheeled object can't move relative to a fixed point unless it's wheels either rotate or skid.
If the rotation of its wheels is negated by a perfectly counteracting conveyor belt then it has to be skidding.
Even if it's freewheeling. Freewheeling and skidding at the same time. Takes a bit of thought to get your head around that, innit.

Sticky rubber skidding on a runway surface equals friction which equals heat.
Do it for a couple of miles with up to 180mph difference between the two surfaces and 300 tons of Jumbo pushing down. Could get messy.

 

[Deleted member 35927]

A wheeled object can't move relative to a fixed point unless it's wheels either rotate or skid.
If the rotation of its wheels is negated by a perfectly counteracting conveyor belt then it has to be skidding.
Even if it's freewheeling. Freewheeling and skidding at the same time. Takes a bit of thought to get your head around that, innit.

Sticky rubber skidding on a runway surface equals friction which equals heat.
Do it for a couple of miles with up to 180mph difference between the two surfaces and 300 tons of Jumbo pushing down. Could get messy.

But we know this is nonsense in terms of a plane moving along the belt. The wheels will always rotate faster and faster.
As long as you're ok with your outcome not existing in the real world then that's fine.

 

[Golf is fun]

A wheeled object can't move relative to a fixed point unless it's wheels either rotate or skid.
If the rotation of its wheels is negated by a perfectly counteracting conveyor belt then it has to be skidding.
Even if it's freewheeling. Freewheeling and skidding at the same time. Takes a bit of thought to get your head around that, innit.

Sticky rubber skidding on a runway surface equals friction which equals heat.
Do it for a couple of miles with up to 180mph difference between the two surfaces and 300 tons of Jumbo pushing down. Could get messy.

This overlooks one key problem - in the real world for the conveyor to counteract the wheels turning it has to move in the direction of the planes travel, for the conveyor to match the speed of the wheels in the opposite direction it has to move in the opposite direction of the planes travel. Therefore to read the question literally the conveyor has to be moving both forwards and backwards at the same time - Schrodinger's conveyor!

Going back to your trundle wheel, stand along side a moving conveyor with a trundle wheel on the conveyor, walk at the exact same speed as the conveyor observe that the wheel is not turning, nor skidding, but you are moving, and the wheel is moving relative to everything in the world except the conveyor and you. This is the only way a conveyor can stop a free wheel turning.

 

[SocketRocket]

Ah no, you need an opposing force equal to or greater than the thrust of the plane in order to hold it stationary. Not a hope in hell are you getting that from the conveyor via the free spinning wheels.

This is a theoretical exercise, we have to accept the conveyor moving in the opposite direction to the engines thrust can match the forward momentum of the airframe. The original specification does not detail the amount of thrust the engines will use or the amount of opposing torque created by the conveyor. In this case we must consider them as equal.

 

[Golf is fun]

For those who live in the real world:

 

[Deleted member 35927]

This is a theoretical exercise, we have to accept the conveyor moving in the opposite direction to the engines thrust can match the forward momentum of the airframe. The original specification does not detail the amount of thrust the engines will use or the amount of opposing torque created by the conveyor. In this case we must consider them as equal.

What you suggested is not how it works. Conveyor has no effect on the thrust produced by the plane , simple laws of motion.

 

[SocketRocket]

For those who live in the real world:

I'm not convinced the wheels are not creating traction with the ground due to the weight of the aircraft on the thin sheet. Also IMO the experiment does not specify the amount of thrust the engines use, of course the plane will move forward if the engines are allowed an amount of force whereby the wheels cannot grip the conveyor surface. I think the original specification is suggesting these will be equal.

 

[SocketRocket]

What you suggested is not how it works. Conveyor has no effect on the thrust produced by the plane , simple laws of motion.

If you walk on an escalator in the opposite direction to it's travel and at the same speed you will remain stationary with respect to the escalator. The thrust is made by your muscles and legs. As long as the speed of the escalator is balanced to the speed of you walking everything is in balance. Things only react differently if that balance is disturbed by your pace or the speed of the escalator.

What simple law of motion is this breaking.

 

[Whereditgo]

The problem with that demonstration is that the wheel speed was not measured. The wheels had to have been rotating faster than the sheet was being pulled in the opposite direction.