Puzzles,conundrums, riddles and thoughts?

[Riffraff]

Censored plane problem and its censored wording.

I ask: Where would the plane get upwards acceleration from?

well air rushes past the wings of the plane. The wings are formed, so that the air passes faster over the top than the bottom. This leads to a lower pressure on the top side of the wing (Bernoulli principle) - and thus to an upward force.

 

[Riffraff]

Ignore anything anyone ever tells you about air pressure. When air passes over an aircraft wing the shape of the wing forces the air to leave the trailing edge with a downward component. Newton's first Law means that the reaction pushes the plane up.

In this problem the wording implies that you have a stationary plane, when in reality you'd get a moving plane, regardless of the conveyor.

hmm i thought it was pressure, and its surely newtons third law you're reffering to.

 

[oagersnap]

Ignore anything anyone ever tells you about air pressure. When air passes over an aircraft wing the shape of the wing forces the air to leave the trailing edge with a downward component. Newton's first Law means that the reaction pushes the plane up.

No, it is air pressure that makes the plane rise.

 

[brennan]

Nope, look at an aircraft wing, they all have the same shape, with a downward sloping trailing edge. If the need was only for a longer upper surface, a simple hump would do the trick.

Helicopters generate fearsome downdraughts, why? because the blade (which is exactly the same shape as the wing on a fixed-wing aircraft) pushes the air down. If the wing was creating a low pressure area above it, the air would be moving upwards.

 

[El_Machinae]

As well, planes can fly upside down for periods at a time.

 

[Sparta]

Look - we're over 2% of the way to tying that physics forum!

I believe that brennan is correct, at least as far as I understand it. Newton's third law and angle of attack play a larger role than the pressure difference exhibited by Bernoulli's Principle (especially in the case of flying upside down of course, as El_Machinae notes).

Here is yet another Straight Dope article on the phenomenon, in case anyone is interested in more details.

WANT MORE RIDDLES!

 

[Erik Mesoy]

Remembered from my book of riddles that I lost a long time ago:

A man is going up an escalator when suddenly he stops. This makes him begin crying. Why?
(It's nothing to do with the man himself)

A professional athlete is found to have committed suicide, obviously by his own hand. The police investigate his house and decide that they have an instigation on their hands upon seeing the postbox. Why?
(No, it's not the letters or lack of them, it's upon seeing the postbox itself.)

 

[Taliesin]

The escalator stops because of a power failure, which causes the man to realise that his mother's dialysis machine will also stop working, causing her death.

 

[warpus]

A man is going up an escalator when suddenly he stops. This makes him begin crying. Why?
(It's nothing to do with the man himself)

Because he gets stuck in the escalator, which is why it stops, which causes him pain?

 

[Irish Caesar]

He stops when he gets to the top because the escalator has come to an end. However, he didn't step off, and the escalator shreds his feet. He cries from the pain.

Although Taliesin's is good, too.

 

[Kan' Sharuminar]

I think Taliesin's is the correct answer, as it's the same one as a book of riddles I read. Though I usually wondered why the hypothetical hospital didn't have a back-up generator for such trivial things as life-support machines.

 

[Taliesin]

Rats, I was just taking a wild guess. Hmm, now I have to go find a suitable puzzle.

 

[El_Machinae]

A professional athlete is found to have committed suicide, obviously by his own hand. The police investigate his house and decide that they have an instigation on their hands upon seeing the postbox. Why?
(No, it's not the letters or lack of them, it's upon seeing the postbox itself.)

a) The flag was up, indicating that he was ready for more mail to be delivered. Who resets their mailbox if they're going to kill themselves?

b) His shipment of Steroids! recently arrived, which makes us wonder what was in the syringe he had injected himself with, if not his steroids (i.e., his murderer sent him a package disguised as his regular package, with a poison inside).

c) His postbox has been smashed by a burned cross, and the football player is black

d) He placed his payment for his electricity bill in the mailbox, for pickup by the mailman

e) there's a trap designed to slash wrists when a hand is put into the mailbox. The poor fellow stumbled home and fell into his bathtub, making it look like a suicide.

 

[Taliesin]

John while Jim had had had had had had had had had had had a better effect on the teacher

Punctuate this sentence to make it grammatical.

 

[Erik Mesoy]

Taliesin was correct. IC's can't technically be correct because it's not as the man is going up.

As for the other one, it's because the mailbox was found to be 3 inches taller than normal. Rats, now I think I may have told it backwards. The man was a basketball player and had been convinced that he was shrinking, since one of his rivals had been heightening the postbox.

Answer a was great, though.

 

[El_Machinae]

John while Jim had had had had had had had had had had had a better effect on the teacher.

 

[Steph]

Nope, look at an aircraft wing, they all have the same shape, with a downward sloping trailing edge. If the need was only for a longer upper surface, a simple hump would do the trick.

Helicopters generate fearsome downdraughts, why? because the blade (which is exactly the same shape as the wing on a fixed-wing aircraft) pushes the air down. If the wing was creating a low pressure area above it, the air would be moving upwards.

No, a plane flies really flies because of the difference of pression between the upside and the downside of the wings.

Bernouilli's principle: pV²/2 + P = cte
So if the speed increases, the pressure decreases.

A wing that goes through air is creating a turbulence: the extrados (upside of a wing) is curved, so the air that flows along needs as to go faster (as it needs to make a longer path than the average of the wings), and thus the pression is lower, the upside is "sucked" upward.
At the opposite, along the intrados, the speed of air is lower, so the pression is greater, the downside is pushed.

The result is the top of the wing is suck upside (main force), and the bottom is push upside (additional force). By combining both forces, the plan is lifted.

 

[Taliesin]

And here's a cute one, while you're working on the hads.

This is a letter bank problem, where the BANK is a word which contains no repeated letters, and the PHRASE is composed only of letters from the BANK, with each BANK letter used at least once.

Oh shed a Tear for Branwell BANK, the literary Lad!
He never won the sort of Fame his elder Sisters had.
Some call the Poppy culpable, or hold the Grape to blame
For Branwell's tragic early Death in Misery and Shame.
I say the Villain was a more insidious Addiction:
Not Opium nor Alcohol, but Melancholy Fiction.

The Cords of Melodrama wrapped around his fevered Brain;
They squeezed from it all Sense, and drove him Gothically insane.
He gave his Soul entirely to picturesque Despair
And prowled the local Moor at night, to sob and tear his Hair.
"Oh, in this darksome, windswept World, what does my poor Life matter?"
He struck a Pose, and cried "A PHRASE", and opted for the Latter.

(His sisters died soon afterwards, but they won great repute,
While he's a name for Jeopardy!, or Trivial Pursuit.)

 

[ainwood]

No, a plane flies really flies because of the difference of pression between the upside and the downside of the wings.

Bernouilli's principle: pV²/2 + P = cte
So if the speed increases, the pressure decreases.

A wing that goes through air is creating a turbulence: the extrados (upside of a wing) is curved, so the air that flows along needs as to go faster (as it needs to make a longer path than the average of the wings), and thus the pression is lower, the upside is "sucked" upward.
At the opposite, along the intrados, the speed of air is lower, so the pression is greater, the downside is pushed.

The result is the top of the wing is suck upside (main force), and the bottom is push upside (additional force). By combining both forces, the plan is lifted.

Actually, that is slightly incorrect. The flaw in that explanation is that "'the air that flows along [the top] needs to got faster (as it needs to make a longer path than the average of the wings".

Better explanations of the issue than I can give are here and here.

In particular:

Some lay versions of this explanation use false information due to lack of understanding the Kutta condition, such as the incorrect assumption that the two parcels of air which separate at the leading edge of a wing must meet again at the trailing edge. There is no reason that a parcel of air on one side of the wing must rejoin a neighboring parcel with which it was originally synchronized on the other side. In fact, the requirement for circulation (see below) in order to generate non-zero lift specifies that parcels must never meet.

 

[Steph]

My explanation is a simplification that doesn't take into account all the forces and effect that could happen on a wing. Note that in what you quote you said "incorrect assumption that the two parcels of air which separate at the leading edge of a wing must meet again at the trailing edge". I didn't say the flows must meet exactly again at the trailling edge , I said it was going faster upside, and slower downside, and as the flows do not meet exactly, it creates a turbulence.

Anyway, I'd trust the aerodynamics and flight mechanism teachers I had during my engineer course at the French Civil Aviation University before wikipedia if you don't mind.