Math trick

[DrWho]

EDIT2 - I changed my mind....

i have no clue... [H]

[Ray Garrison]

Wow...

That guy had a gift. For such a seemingly simple question...

Google feynman sprinkler under water

You'll find advanced physics departments at various universities have

been arguing this for years, and there appears to be no diffinitive

answer to the question.

Wow...

[Jay481985]

Well done Gil. Here is another problem from

Richard Feynman that he used to start arguments among physicists at

parties.

It's obvious that the sprinkler nozzles below will

rotate clockwise when water flows out of the nozzles under

pressure. If you submerge the sprinkler in a tank of water

and suck water into the sprinkler (reversing the flow), which way

will the nozzles rotate?

it would not move...

water is denser than air as it pushed the air. Since the environment is

trying suck is water and it has the same density all around it would

not move.

[Fish]

That cracked me up.Steelers smiley that is.

[sputnik]

Wow...

That guy had a gift. For such a seemingly simple question...

Google feynman sprinkler under water

You'll find advanced physics departments at various universities have

been arguing this for years, and there appears to be no diffinitive

answer to the question.

Wow...

Feynman was truly amazing and a genius in every sense of the

word. This was a somewhat famous teaser that he would toss out

and then argue convincingly that the nozzle would go a certain

direction and then he could change his argument to convince just about

anyone that the nozzle would go the other way.

But here is the explanation:

When water is jetting out of the nozzle, it has a single direction and

magnitude (relative to the nozzle) that results in a force and the

momentum of the water jet is equal to the momentum that spins the

nozzle. But in the case of reverse flow (suction), the flow of

water toward each nozzle point is radial and isotropic (of equal

magnitude in all directions) so that all the forces cancel each other

out and the resultant force on the nozzle is negligible and so there is

no rotation. The nozzle may shake but won't rotate. The

story goes that Feynman was banned from the cyclotron lab at Princeton

after he pressurized a water bottle with a sprinkler in it that

subsequently exploded.

[Ray Garrison]

Surprizingly enough, the answer is really not so simple...

The Feynman inverse sprinkler problem: A detailed kinematic study

American Journal of Physics -- April 1991 -- Volume 59, Issue 4, pp. 349-355

http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=AJPIAS000059000004000349000001&idtype=cvips

Abstract

This paper represents an extension of the results of an original paper

on the subject [Am. J. Phys. 57, 654657 (1989)]. Here an experiment,

originally performed by Titcomb et al., to demonstrate the inverse

sprinkler effect is described and analyzed. It is shown that, in a

water reservoir, the angular momentum of the sprinkler head is

approximately equal and opposite that of the fluid in the sprinkler

arms. However, in an air reservoir, one must consider the effects of

both turbulence and the motion of the reservoir. Both effects result in

a steady-state acceleration of the inverse sprinkler in a direction

opposite that of the normal sprinkler. Also presented is a more

detailed analysis than the one given in the original paper to explain

the steady-state behavior of the inverse sprinkler.

[DrWho]

Ya know....this is one of those times where you could argue the physics all you want, but it would be so much easier to just try it and see what happens [] You know, that whole experimental thing that drives all the theories we're using to come to a conclusion, lol []

[Jay481985]

I concluded my answer just because the water is pushing across air.

Kinda like a water bottle rocket where the water is pressured and is

pushing the air. In reverse since there is equal suction, and equal

density, it would not move. I did not google it I just pondered for a

bit.

[WMcD]

Hmm, maybe the joke is on me. I should Google before thinking.

BTW I have noted that when you put a garden hose nozzel in a pool the reaction is reduced, but not eliminated. Also, when I was a little kid I did fill up the little pool with a rotating sprinkler. It did spin underwater, to my recall. This makes sense because Sea-Doos use underwater jets.

I can see that part of the issue in suction on the lawn sprinkler is that we reverses the flow. But we have not reversed the geometry of the conical converging nozzle.

So I wonder what happens if we do that. Put a diverging nozzle on the arms. Envision Horn Ed's ears (the avitar) on the ends. Smile

I would think that the studies are not saying we can never devise a suction operated underwater sprinkler turbine.

If we put a pin wheel in a bathtub drain, it would spin. That is how the Hoover Dam makes electricity.

Gil

[sputnik]

Hmm, maybe the joke is on me. I should Google before thinking.......

Gil

That's an interesting point. I wonder if Google will impact they

way we think and retain knowledge the way handheld calculators seem to

have affected the way we do math. It's too easy just to Google up

an answer for just about anything rather than flex our brains a bit.

Maybe it frees us to think in more abstract terms but does it also

create an intellectual dependence on technology over analytical

thinking? Or just thinking for the fun of it. That's one of

the reasons that I admire guys like Richard Feynman. He won the

Nobel Prize for his work in quantum electrodynamics among his other accomplishments that few people

could really understand and yet he still pondered the physics of something

as simple as a lawn sprinkler.

Anyway, here is a neat math problem I found (using Google) when helping my nephew with his homework.

Three balls are placed inside a cone such that each ball is in contact

with the edge of the cone and the next ball. If the radii of the

balls are 20 cm, 12 cm, and r cm respectively, what is the value of r?

[Jay481985]

4? and no google

[sputnik]

4? and no google

Good try using the pythagorean theorem (that would've been really cool) but that's not the right answer. []

[Jay481985]

is it a whole number?

[sputnik]

No, it's not whole number.

Added:

Speaking of the pythagorean theorem, I think this is the coolest proof of it.

[customsteve01]

I would say 7.2

But I bet thats not right.

Steve

[sputnik]

I would say 7.2

But I bet thats not right.

Steve

You're wrong (about being wrong). 7.2 is the right answer. []

Using similar triangles, (12 + r)/(12 - r) = 32/8 = 4.

? 12 + r = 48 - 4r

? 5r = 36 ? r = 7.2 cm.

[Jay481985]

bah

[customsteve01]

Wow thats cool, but I got the answer a lot easier.

20/12=1.6666666666666667

12/1.666666666666667=7.2

my way was way too easy thats why I thought I was wrong.

Steve

[Jay481985]

heh I did it the opposite way and got a .7 and mutiplied that by 12 and

got 8.4.....and then halfed it 4.2 but thought oh well 4...... What

sputnik did was a three piece system using pythagoream's theorem (which

btw is still a theory that works in most cases). Bah I forgot to do it

that way. I wasn't thinking that that cone would be a similiar triangle.

[Ray Garrison]

Here's a puzzle American Can used to give their managers years ago -

was part of a packet with the famous "Connect these 9 dots with four

lines without lifting your pencil from the page" puzzle...

Here is a sequence of digits. What pattern are they following, and what is the next number(s) in the sequence?

8 5 4 9 1 7 6

brief aside - I felt a little depressed when I saw a few references to

thrust moving objects because the exiting mass was "pushing" on

something... rockets do fly in outer space, ya know... what happened to

basic science classes?