Stupid physics question

[DaGaffer]

Well, as Tilda demonstrates, a conveyor belt would only work if the propulsion system directly involved the wheels, i.e. it relied on frictional contact with the ground, like a regular car.

However, most planes don't have this form of propulsion, in a frictionless environment the external propulsion (propeller or jet) would still move the plane along the conveyor, the wheels would just move faster.

Hypothetical conveyor or not, it is a meaningless and useless device for controlling the relative speed of the plane, so the question is either a trick one, or it is flawed by lack of knowledge on how friction physics works.

It's like the "Cowboy rides into town on Friday, stays three days, and leaves on Friday" sort of question, it is just plain bollocks.

It doesn't matter. You're all thinking like engineers not physicists. Everyone's diving into the mechanics of how a conveyor belt would counteract a plane's forward motion, which is utterly besides the point. The statement says it does, that's all you need to know for the purposes of a thought expiriment.

 

[Xgkkp]

I dont think you you have comprehended the facts....

hahahahahahahaha

It doesn't matter. You're all thinking like engineers not physicists. Everyone's diving into the mechanics of how a conveyor belt would counteract a plane's forward motion, which is utterly besides the point. The statement says it does, that's all you need to know for the purposes of a thought expiriment.

So a physicist wouldn't look into the mechanics of a problem? Incidentally the question doesn't claim that the conveyor belt 'counteracts the planes forward' it claims that it opposes the speed.

Each action has an equal and opposite reaction. Therefore, nothing ever can possibly move.

 

[ECA]

Someone should retitle this "Horrible Physics".

 

[xane]

That said, this question has done a very good job around the internet of showing how people can't cope with moderately simple systems outside of their immediate experience, and trust their knee-jerk answers more than logic.

What it demonstrates is the lack of description in the question.

It is clear that the question implies a machine that is intended to keep the relative speed of the plane at 0 mph. It is with that in mind that most of the answers regarding air speed were made.

Either this implication was cleverly done to distract, as in the real world such a machine would _not_ keep the plane at 0 mph, or the question was set by someone who does not understand the physics and intended such a machine but got it wrong.

 

[Tom]

It doesn't matter. You're all thinking like engineers not physicists. Everyone's diving into the mechanics of how a conveyor belt would counteract a plane's forward motion, which is utterly besides the point. The statement says it does, that's all you need to know for the purposes of a thought expiriment.

The conveyor belt does not counteract the aircraft speed - it simply matches in, in the opposite direction.

 

[Louster]

The statement says it does, that's all you need to know for the purposes of a thought expiriment.

Except it actually doesn't - but the fact that you can interpret this way is why it's achieved such debate. It says the speed of the conveyor is equal to the plane's speed. That's all it says. This does not imply that the conveyor will counter the plane's motion, as the conveyor is not 'connected' directly to the plane's motion. In fact, it makes no logical sense - what 'speed' is it defined as countering, then, if it's motionless? If the plane is motionless, then surely the conveyor is also motionless. (Motionless in the other direction, no less!)

 

[Tom]

This works exactly the same as running on a treadmill...you run at exactly the same speed as the treadmill and you have no wind blowing in your face.

You remain stationary in relations to everything around you....but work very hard....

The plane would therefore not lift off the ground as there is no air moving past it.

Fair enough, but forward motion on the plane is not achieved by sending power to the wheels.

Imagine your treadmill, with a bike on the treadmill and you stood at the side.

The treadmill moves at 100mph, but you're holding onto the bike and not allowing it to move. How fast does the bike go? The wheels are going at 100mph, but its not going anywhere.

Now you walk forwards at 5mph. The belt is doing 100mph, the wheels are doing 105mph, the bike is doing 5mph.

 

[Xgkkp]

Ok explain it to me then fuckwit

I think if the sane and intelligent people posting in this forum haven't persuaded you then I doubt god himself (or a deity of your choice) could explain it to you.

What it demonstrates is the lack of description in the question.

It is clear that the question implies a machine that is intended to keep the relative speed of the plane at 0 mph. It is with that in mind that most of the answers regarding air speed were made.

Either this implication was cleverly done to distract, as in the real world such a machine would _not_ keep the plane at 0 mph, or the question was set by someone who does not understand the physics and intended such a machine but got it wrong.

I agree about the fuzziness of the question, it encourages you to jump straight to a wrong conclusion. I believe it was carefully constructed to confuse people initially and make them think more deeply about a problem.

Rather than immediately jumping to a conclusion and vehemently defending it with false-logic.

 

[dysfunction]

I think if the sane and intelligent people posting in this forum haven't persuaded you then I doubt god himself (or a deity of your choice) could explain it to you.

So you cant explain it in your own words then.....

 

[Rubber Bullets]

A plane that required wheeled power to attain sufficient airspeed to takeoff would be doomed to failure. As soon as it took off it's forward impetus would be lost and it would come back down again.

Therefore planes rely on engines that move large chunks of air from infront of them to behind them. Newton tells us that for every action there is an equal and opposite reaction and so as the air is moved backwards he plane moves forward.

In order to take off the power of the engines must be sufficient to move the plane forwards fast enough that the lift generated (proportional to speed) is greater than the planes weight. To do this the engine has to overcome the friction of the wheels on the ground and the air passing over the body.

So in this case as the engines will still be abe to move the plane forward regardless of the ground moving or not. All that will happen is that the wheels will be rotating twice as fast as they would normally at the takeoff speed. The engines would have to be able simply to overcome the extra friction that results from this.

The analogy of the treadmill is totally false. The queston states that the conveyor is travelling backwards at the same rate that the plane is travelling forwards. This is not the case on a treadmill, if it were we'd fall off the front!

I would hope that the wheels on any plane I flew on would be sufficiently over engineered to take speeds of twice the normal takeoff speed, but I have no idea if this is the case or not. The wheels would not need to be perfectly frictionless, but the less they have the better, again in line with most aircraft wheels I guess.

I am amazed that in the poll more people have the wrong answer than the correct one.

RB

 

[Xgkkp]

So you cant explain it in your own words then.....

I can't explain it in my own words that you could understand, no, because if many other people have explained why and you still don't get it there is no point in trying.

Nobody is ever going to win this because the people who say it won't just dismiss anything countering their arguments as 'not specified in the problem'.

Oh hey, gravity isn't specified in the problem, so yes it does take off because it just floats up!

 

[Jupitus]

Xgkkp - welcome to the boards - I assume you are a visitor from the other forum Tom mentioned originally I'm not clear as to whether you are in the yes or no camp... please confirm.

Anyway - the plane only has forward motion relative to the conveyor, not to the air, hence no air flow across the wings.

The principle of lift is Benoulli's Principle and it works on the basis that the air flowing over the top, curved surface of the wing moves faster (because it has further to travel) than the air flowing across the flat underside of the wing. This speed differential creates an area of low pressure relative to the bottom surface and hence creates an upward pressure on the wing itself, i.e. lift. If the plane is stationary relative to the air, which in this scenario it is, then it will not take off.

 

[Louster]

I am amazed that in the poll more people have the wrong answer than the correct one.

RB

And by a massive proportion, too. Tells you something about this place, I guess.

 

[xane]

I agree about the fuzziness of the question, it encourages you to jump straight to a wrong conclusion. I believe it was carefully constructed to confuse people initially and make them think more deeply about a problem.

But then you get the issues that Tilda brought up, there is no mention of frictionless wheels so it will be assumed that the movement of the conveyor belt will some drag affect on the plane, whether this drag is enough to prevent the plane reaching takeoff speed is a matter of many unmentioned variables; the weight of the plane, the heat capabilities of the wheels or indeed the takeoff speed of the plane, etc.

I go for the "misunderstood physics" argument, and I assumed that most people did the same and assumed that the machine was intended to keep the relative speed of the plane at 0 mph.

 

[dysfunction]

I can't explain it in my own words that you could understand, no, because if many other people have explained why and you still don't get it there is no point in trying.

Nobody is ever going to win this because the people who say it won't just dismiss anything countering their arguments as 'not specified in the problem'.

Oh hey, gravity isn't specified in the problem, so yes it does take off because it just floats up!

jebus you are painful

 

[Xgkkp]

Xgkkp - welcome to the boards - I assume you are a visitor from the other forum Tom mentioned originally I'm not clear as to whether you are in the yes or no camp... please confirm.

Anyway - the plane only has forward motion relative to the conveyor, not to the air, hence no air flow across the wings.

Thank you very much for the welcome, though unfortunately I am going to have to then disagree with you, I am in the 'Yes' camp, and I came through a different poster, not Tom.


Nowhere in the question does it specify that the plane is not moving relative to the air, (in fact the question claims that the plane moves) it just specifies that the plane is *moving forwards* at the same speed that the conveyor is moving backwards. This would be a problem except for the fact that the motion of the plane itself is not directly related to the motion of it's wheels, the motion of which is related directly to the conveyor belt.

A similar analogy might be that of a hovercraft 'sailing' (or whatever you call what a hovercraft does) up a river.

 

[DaGaffer]

hahahahahahahaha


So a physicist wouldn't look into the mechanics of a problem? Incidentally the question doesn't claim that the conveyor belt 'counteracts the planes forward' it claims that it opposes the speed.

Each action has an equal and opposite reaction. Therefore, nothing ever can possibly move.

No, that would be the subject of a separate experiment. And clearly the last line of your statement is utterly irrelevant. Nothing would ever move if everything had the same mass, which has no bearing on this.

*edit* even assuming you do include the wheels in the model, the plane still won't take off; it needs to reach a certain velocity when its on the ground in order to take off, if a conveyor was working against this it would increase the required ground speed needed to take off. The plane may move forward, but not quickly enough.

 

[Jupitus]

Thank you very much for the welcome, though unfortunately I am going to have to then disagree with you, I am in the 'Yes' camp, and I came through a different poster, not Tom.


Nowhere in the question does it specify that the plane is not moving relative to the air, (in fact the question claims that the plane moves) it just specifies that the plane is *moving forwards* at the same speed that the conveyor is moving backwards. This would be a problem except for the fact that the motion of the plane itself is not directly related to the motion of it's wheels, the motion of which is related directly to the conveyor belt.

A similar analogy might be that of a hovercraft 'sailing' (or whatever you call what a hovercraft does) up a river.

So - the wheels are not connected to the plane?

 

[Xgkkp]

But then you get the issues that Tilda brought up, there is no mention of frictionless wheels so it will be assumed that the movement of the conveyor belt will some drag affect on the plane, whether this drag is enough to prevent the plane reaching takeoff speed is a matter of many unmentioned variables; the weight of the plane, the heat capabilities of the wheels or indeed the takeoff speed of the plane, etc.

I go for the "misunderstood physics" argument, and I assumed that most people did the same and assumed that the machine was intended to keep the relative speed of the plane at 0 mph.

Agreed, there will be some effect on the plane from wheel drag, however wheels are designed to travel faster than the thing to which they are attached so it can pretty much be ignored. The effect will be that the plane would have to give slightly more thrust (or take a slightly longer distance to take off), that is all - in no way is friction of the wheels a dominant factor.

I do, however disagree about the intent of the question - I believe it was intended to mislead people into assuming what you assumed - otherwise what is the point of asking the question? Maybe someone should try to track down the origins, and chances are it started from a professor teaching a physics class.

 

[Xgkkp]

So - the wheels are not connected to the plane?

Physically, yes (well, through an mediator such as oil) but not mechanically - their motion isn't connected.

And clearly the last line of your statement is utterly irrelevant.

Yes, that was the point. I was attempting to demonstrate the logic of (some of) the no camp.

Also please people stop explaining aircraft lift, we all know that 0 air velocity = 0 lift and that isn't the question, it's getting annoying.

 

[Jupitus]

Can one of the yes camp please explain where the airflow over the wings comes from?

 

[Rubber Bullets]

The forward velocity of the aircraft.

RB

 

[xane]

I do, however disagree about the intent of the question - I believe it was intended to mislead people into assuming what you assumed - otherwise what is the point of asking the question?

You can muse with the "intent" of the question forever.

plane. n.
1. A carpenter's tool with an adjustable blade for smoothing and leveling wood.
2. A trowel-shaped tool for smoothing the surface of clay, sand, or plaster in a mold.

 

[Xgkkp]

The forward velocity of the aircraft.

This

 

[Jupitus]

... and why the plane has any forward velocity when the surface on which it is standing has an equal and opposite velocity?

 

[xane]

Can one of the yes camp please explain where the airflow over the wings comes from?

The answer is not about the airflow, it is about whether the plane would move forward or not.

 

[Xgkkp]

You can muse with the "intent" of the question forever.

So the conveyor belt is made of wood? How sharps is the plane? Is it too heavy? Is that why it can't be taken off?

It is a mystery.

 

[Xgkkp]

... and why the plane has any forward velocity when the surface on which it is standing has an equal and opposite velocity?

Because the motion of the plane has nothing to do with the surface on which it is standing.

 

[xane]

It is a mystery.

As would why someone would devise such a machine if it is theoretically useless.

 

[Jupitus]

Because the motion of the plane has nothing to do with the surface on which it is standing.

This would be a problem except for the fact that the motion of the plane itself is not directly related to the motion of it's wheels, the motion of which is related directly to the conveyor belt.

So... the wheels are connected to the plane, and the motion of the wheels is 'related directly to the conveyor belt' and yet you argue that although the conveyor is moving at an equal speed but in the opposite direction the plane would still have forward velocity...