Skill's weekend teaser

[Skill Leverage]

I hate to burst anyones bubble, but the reason why there are so many incorrect answers out there is that the question was phrased incorrectly.

The link provided by Candles has the theoretical set up as,

"If an airplane is on a large conveyor belt and is trying to take off by exerting the thrust needed to move it forward at 100 knots, and the conveyor belt starts moving backwards at 100 knots..."

In this situation, the conveyor belt has a constant linear velocity of 100 knots. The airplane would indeed move forward and could take off normally.

Skill Leverage phrased this situation,
"Imagine a 747 sitting on a very large conveyor belt. The belt has the same dimensions as a runway at an airport*, and is set up to exactly match the speed of the plane's wheels, moving in the opposite direction. What will happen?"

Unfortunately for Skill Leverage, he has the wrong theoretical set up and has stated that the conveyor belt moves at the same (I am assuming linear) speed as the wheels. The airplane could not move in this case even though the wheels do not drive the plane forward. In this theoretical situation, the conveyor will always match the speed of the wheels; ie, it is not a constant speed as in the link above. Hence, anytime forward thrust occurs and the wheels start turning, the conveyor belt matches this speed and cancels the momentum. There is one other caveat, however, and that is where Skill Leverage said that the conveyor is moving in the opposite direction. If the wheels were considered as moving clockwise, then a counterclockwise moving conveyor would still yield no linear movement relative to the wind and produce no lift. If, however, the wheels and conveyor movement were analyzed in a linear fashion, then the plane would actually have a relative movement twice as fast as the linear velocity of the wheels. This would result in the plane able to take off in half the normal distance. Either way, the question is phrased incorrectly for any of the answers to be correct.

You may all begin the debate anew.

Ha. Ha. Ha.

Maybe all planes should just have someone spin their wheels really really fast, then they could take off from someone's bedroom...

You, Sir, are the biggest idiot of them all. The question is phrased exactly as it should be; you just cannot understand it.

 

[ezreddy]

Ha. Ha. Ha.

Maybe all planes should just have someone spin their wheels really really fast, then they could take off from someone's bedroom...

You, Sir, are the biggest idiot of them all. The question is phrased exactly as it should be; you just cannot understand it.

Here is a googled site that explains at the bottom how you incorrectly phrased your question:

The Straight Dope: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

The way you phrase your question sets up an impossible situation which cannot occur. Had you stated that the conveyor moves at the same speed as the airplane, then you would have been correct.

I am sure you found this somewhere on the Internet and wanted to amaze your trading buddies. Unfortunately, you remind me of the kids who cheated on tests, but weren't even smart enough to cheat off of someone who had the correct answer.

 

[Skill Leverage]

Here is a googled site that explains at the bottom how you incorrectly phrased your question:

The Straight Dope: An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

The way you phrase your question sets up an impossible situation which cannot occur. Had you stated that the conveyor moves at the same speed as the airplane, then you would have been correct.

I am sure you found this somewhere on the Internet and wanted to amaze your trading buddies. Unfortunately, you remind me of the kids who cheated on tests, but weren't even smart enough to cheat off of someone who had the correct answer.

Unfortunately for you matey, I took the question directly from a book. It does not matter whether the belt moves at the speed of the wheels, the speed of the plane or the speed of my laughter exiting my mouth; the plane still takes off. Get back to me when you understand that one.

 

[Skill Leverage]

Taken from the very link you provide:

However, some versions put matters this way: "The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation."...The question thus stated asks the impossible -- simply put, that A = A + 5 -- and so cannot be framed in this way. Everything clear now? Maybe not. But believe this: The plane takes off.

The paradox he is referring to is entirely irrelevant to the problem, and is entirely a matter of semantics. The plane takes off. If you want to get into a debate about the nature of infinity, we can start a new thread. Otherwise, thanks for being the guy who has nothing to contribute other than picking small holes, due to complete lack of original thought.

We can all google 'plane on a treadmill' and say 'ooh, here's someone saying SL has made a tiny mistake'... only a few of us can actually think on our own feet though.

Have a nice day.

SL

 

[TheBramble]

Bramble,

So do you believe that in this theoretical situation, the jet engines would have no affect on the plane whatsoever? Turn the jet engines to full whack and the plane won't move anywhere?

Sam.

Sam, given the limiting constraints set in post #1 (wheels/conveyor) yes.

That is the only difference between a normal 747 at the end of the runway ready for take off and the one Skills is postulating.

He's set the teaser himself and missed this limiting constraint upon which this entire solution depends.

 

[TheBramble]

Imagine a 747 sitting on a very large conveyor belt. The belt has the same dimensions as a runway at an airport*, and is set up to exactly match the speed of the plane's wheels, moving in the opposite direction.

IS NOT THE SAME AS SAYING:

The motion of the belt counteracts the forward motion of the plane.

Erm, it's exactly the same. Think it through.

 

[MrGecko]

Bramble - please tell us what force is preventing the plane from moving forwards from the thrust generated by the engines.

If the plane doesn't move, something must be holding it back. What?

 

[Skill Leverage]

For the love of God, just lock the thread please. He's never going to get it, we could be here forever and it will inevitably get out of hand.

 

[porph]

What an obnoxious thread to find on a monday morning.

 

[Skill Leverage]

What an obnoxious thread to find on a monday morning.

Don't let the door hit you in the **** on the way out

 

[Skill Leverage]

All joking aside, it has been fairly obnoxious, massively down to me I'm sure but, there's only so many times you can explain something before you get exasperated. As I said, thread should be locked imo, there's nothing more to say.

 

[raysor]

Imagine a 747 sitting on a very large conveyor belt. The belt has the same dimensions as a runway at an airport*, and is set up to exactly match the speed of the plane's wheels, moving in the opposite direction. What will happen?

Same rules as before please; no explanation for a little while, no Googling, everyonerich you are banned from participating**.

SL

*To save confusion, the conveyor belt is the same length as the distance required for a 747 to take off from a normal runway; the frictional co-efficient of runway tarmac and the belt are identical, all other variables such as wind and atmospheric conditions can be ignored.
**I'm just kidding (but not really).

Are the planes engines running?

 

[candles]

plane on a conveyor belt you say?

 

[Skill Leverage]

All material I have given in this thread can now be dismissed; I just smacked myself in the face opening a door, and as such am not allowed to pass comment on anything.

 

[TheBramble]

Bramble - please tell us what force is preventing the plane from moving forwards from the thrust generated by the engines.

If the plane doesn't move, something must be holding it back. What?

Just your comprehension it would appear Mr. G. LOL.

The only thing that prevents the plane moving forward is the constraint placed upon it by Skills that the motion of the wheels is exactly matched by the motion of the conveyor. While we hold that as a given, the plane isn't going anywhere.

 

[Skill Leverage]

Bramble you have to understand that rotational motion of the plane's wheels imparts no lateral force on the plane. That's the point you're not getting, and you don't seem to want to get it so there's really no point in us continuing this debate.

 

[TheBramble]

In all seriousness, after conceding defeat in being able to explain myself yesterday I only came back to address Trendie's inputs as he is an intelligent guy, an old friend and I take the extra effort on that score alone.

Having done that, I really am out of ammo and energy on this one.

Can't wait for next week's.....

 

[Mr. Charts]

Oh, please don't close it.
It was just beginning to catch my interest.

 

[Skill Leverage]

Lol there will be no next week's sadly, as my future colleagues/bosses are probably reading this thinking 'what an obnoxious dick'.

SL

 

[Mr. Charts]

Rest and re-arm, Tony
I also certainly want to hear more of what ezreddy might have to say.
Richard