Stupid physics question

[old.user4556]

I'm not giving it up at all, i'm sticking by my opinion.

If a conveyor belt matches the forward speed of the plane to stop it reaching a takeoff airspeed, then it'll never take off. It's that simple (for me).

 

[Louster]

The engine is not on the ground. The wheels are.

I think to be ultra-clear (as a car's engine "is not on the ground" either) you need to point out (for the umpteenth time) that a plane's engine does not depend on making the wheels spin round to propel the machine forwards, and hence is not 'mechanically connected' to the ground, whereas a car's is. But I'm not real sure there's any hope at this point.

 

[Wij]

If a conveyor belt matches the forward speed of the plane to stop it reaching a takeoff airspeed, then it'll never take off. It's that simple (for me).

Your premise is misguided.

You cannot go from

Premise 1: "Conveyor Belt matches forward speed of the plane"
Premise 2: Something you've not told us

to

Conclusion: "Plane never reaches speed X"

...without a reason why. We have explained that there is no magical 2nd premise to support your conclusion in the case of an aeroplane unless the brakes are on.

 

[Wij]

I think to be ultra-clear (as a car's engine "is not on the ground" either) you need to point out (for the umpteenth time) that a plane's engine does not depend on making the wheels spin round to propel the machine forwards, and hence is not 'mechanically connected' to the ground, whereas a car's is. But I'm not real sure there's any hope at this point.

Yes, that's a related but slightly different point. Hopefully my previous post made this point also.

 

[Tom]

NO - THAT IS HOW A CAR WORKS !!!

The example where the man doing the pulling is not on the carpet was perfect. The engine is not on the ground. The wheels are. Similarly the bloke is not on the carpet. The box is.

*will thread to be closed*

Stop trying to close my thread you ruddy great woofter!

 

[old.user4556]

This is where I disagree (except with Louster/Wij's car engine point).

The engine doesn't turn the wheels - correct and fact (totally agree with you there). However, the wheels MUST turn to allow the plane to move in a forward motion at 160 + mph (to reach airspeed of 160 mph to take off) to get off the ground.

If the ground moves below the plane in such a way that it cancels out its forward motion, then yes it will continue to get faster but so does the conveyor; so take off speed is never reached and the plane doesn't take off.

The plane will reach 500 mph as it continues to accelerate, but it still has a 0 mph air speed and it doesn't take off.

 

[Tom]

lol.

*cackles with glee*

Someone go over to the DaoC lot and get them to post here.

 

[Louster]

The movement of the conveyor belt only directly affects the rotational speed of the wheels! The wheels are not connected to the motion of the plane, and hence this is totally irrelevant to the forwards velocity of the plane as a whole.

 

[old.user4556]

I am making one rather large assumption not described in the original text:

"The problem proposes that the conveyor is able to counteract the plane's forward propulsion."

I am assuming the above, that the conveyor is exactly counteracting the planes forward motion. Thus, in that instance, the plane will never move relative to the air in that perfect and balance example.

Is that the case?

*shrug*

Bed I think . My next question is - how many of you will email people that problem at work tomorrow?

 

[Tom]

I am making one rather large assumption not described in the original text:



I am assuming the above, that the conveyor is exactly counteracting the planes forward motion. Thus, in that instance, the plane will never move relative to the air in that perfect and balance example.

Is that the case?

*shrug*

Bed I think . My next question is - how many of you will email people that problem at work tomorrow?

No. That is not the case. Read the question again.

 

[Xgkkp]

To the No's: What would happen with a hovercraft going UP a river? What about a hovercraft on a conveyor belt?

It's exactly the same situation.

 

[old.user4556]

No. That is not the case. Read the question again.

I have, it's not the clearest and the question does pose you to at least make an assumption or two.

 

[old.user4556]

The question to me came across as implying this is a ‘magic’ conveyor belt, that can in fact keep the plane in place relative to the ground (the idea that this isn’t how it would work in reality was irrelevant), in which case, of course it wouldn’t take off.

Which is what I (and a lot of others on the web) thought.

 

[old.user4556]

To the No's: What would happen with a hovercraft going UP a river? What about a hovercraft on a conveyor belt?

It's exactly the same situation.

A hovercraft isn't on wheels though, so a converyor belt can't ever cancel out the forward motion of the hovercraft where as a conveyor belt (if it were very special) could cancel out rotating wheels.

 

[old.user4556]

Quadruple posting llama!!

http://blog.dankim.com/2006/02/10/the-plane-and-the-conveyor-belt/

The problem reworded in two distinct scenarios from its original vague form, thus proving that both sides of the argument are now correct.

Amen.

 

[Louster]

Haha, except one of the assumptions is rendered completely invalid by dint of it being "magic". The first interpretation only works if you don't understand physics - it's the assumption you're intended to jump to, but it makes no logical sense. So, no. Both sides of the argument are not, in fact, correct.

 

[Will]

Haha, except one of the assumptions is rendered completely invalid by dint of it being "magic". The first interpretation only works if you don't understand physics - it's the assumption you're intended to jump to, but it makes no logical sense. So, no. Both sides of the argument are not, in fact, correct.

You must spread some Reputation around before giving it to Louster again.

 

[Wij]

Quadruple posting llama!!

http://blog.dankim.com/2006/02/10/the-plane-and-the-conveyor-belt/

The problem reworded in two distinct scenarios from its original vague form, thus proving that both sides of the argument are now correct.

Amen.

That is still wrong. The only thing that is counteracting the engines is friction on the wheels. Once the jet engines kick in (and the brakes aren't on) the friction force is DWARFED by the engine thrust. Is scenario 1 assuming that the wheels are rusty ? Or is it assuming that the conveyor is running several times faster than the plane would otherwise be moving therefore the force of friction is multiplied by this amount exactly canelling it ? If the latter is what it is stating then that is VERY far from the original problem.

It's good that you are moving towards admitting you were wrong G. Just go the whole way. You'll look much cooler



*kill thread*

 

[Louster]

Wij, it looks like it isn't giving any actual reason as to how the conveyor counteracts the movement of the plane, just that "it does because that's what the question says". Which it doesn't, really.

Edit: ah in fact, as you say, that link is still misleading. I'd assumed the second rewording was going to be the rational one, but it turns out it actually isn't. "Albeit slowly", great. So much for understanding the confusion. Zuh.

 

[Wij]

It said it moves the same speed as the plane in the opposite direction which I've already pointed out makes no sense. I'm being generous even arguing any further.

*kill Silverscale Drakeling*

 

[Draylor]

What the (*&^%$ is going on here?

16 pages and we're still on the original subject? Theres rules against this sort of thing on FH.

 

[Louster]

It said it moves the same speed as the plane in the opposite direction which I've already pointed out makes no sense.

(Actually, I tried to explain that first, sucka!)

 

[Wij]

(Actually, I tried to explain that first, sucka!)

Then I missed your point dud0r

 

[Sar]

I read it as the plane being on a conveyor belt / treadmill. If that's the case then it wouldn't take off, as has been said with their being no airflow over the wings.

If it isn't a treadmill / conveyor belt then fuck knows as it's worded badly then

 

[Wij]

I read it as the plane being on a conveyor belt / treadmill. If that's the case then it wouldn't take off, as has been said with their being no airflow over the wings.

If it isn't a treadmill / conveyor belt then fuck knows as it's worded badly then

*kill Sar*

If I come back tomorrow night and people are still arguing then I'm going to suicide-bomb Heathrow.

 

[Sharma]

So who are you willing all your l00b to then Wij?

 

[Paradroid]

If I come back tomorrow night and people are still arguing then I'm going to suicide-bomb Heathrow.

...well said, accept the physics people and move on! (like the plane)


: /



For people trying to get their heads around this, try imagining the opposite scenario with the plane landing on the conveyor belt. According to some, the plane should stop dead as soon as it touches the runway - which would be impossible.

 

[Danya]

To the no camp: IT'S NOT A FUCKING CAR!

 

[Danya]

Tilda, I've read the thread and given it careful consideration.

Here is the (or a) proper physics explanation:

http://forum.physorg.com/index.php?showtopic=2417&st=1155&#entry42611

Again, the plane will never fly.

He's wrong. He's changed the problem.

But the treadmill is STILL pumping out 10,000n!

It's not, when you take the brakes off the treadmill maintains speed not force, so drop to 60n.

 

[old.Tohtori]

You're all missing the big question here.

Could this , obviously huge, conveyor belt be used to remove all the excess fat in americans at the same time?