Feynman's Inverse Sprinkler Problem...

[Kevin Marshall]

Okay, here's a question: If you have a sprinkler and put it
underwater so it sucks in water instead of squirting it out, would it
rotate in the same direction as before, in the opposite direction?
This is the same problem from "Surely You're Joking, Mr. Feynman" that
a lot of you may be familiar with. I've heard arguments for all three
sides, and they all sound pretty good. Feynman tried it, and said it
didn't move at all (right before it exploded), but my physics
professor this semester said he tried it too and it moved in the
opposite direction, and Feynman was wrong. Does anyone know which way
is the correct way? Has anyone else tried this?

Kevin Marshall
kbm...@psu.edu

[Alan "Uncle Al" Schwartz]

At face value one can make an excellent case for the inverse sprinkler
rotating in either direction. That, plus Feynman's experiment, make the
direction of rotation obvious.

--
Alan "Uncle Al" Schwartz
Uncl...@ix.netcom.com ("zero" before @)
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[ale2]

In article <58pd7e$3...@sjx-ixn8.ix.netcom.com>

Alan \"Uncle Al\" Schwartz <uncl...@ix.netcom.com> writes:

> At face value one can make an excellent case for the inverse sprinkler
> rotating in either direction. That, plus Feynman's experiment, make the
> direction of rotation obvious.

Feynman's experiment only gives an upper limit to the effect, love to
see a proof that there is no rotation. Al?

[Nathan M. Urban]

I seem to recall seeing a number of articles on the inverse sprinkler
problem in the American Journal of Physics, though I didn't read them.
You might want to look there.
--
Nathan Urban | nur...@vt.edu | Undergrad {CS,Physics,Math} | Virginia Tech

[Martin Gelfand]

In article <58pfjf$t...@r02n01.cac.psu.edu> al...@psu.edu (ale2) writes:
>In article <58pd7e$3...@sjx-ixn8.ix.netcom.com>
>Alan \"Uncle Al\" Schwartz <uncl...@ix.netcom.com> writes:
>
>
>> At face value one can make an excellent case for the inverse sprinkler
>> rotating in either direction. That, plus Feynman's experiment, make the
>> direction of rotation obvious.
>
>Feynman's experiment only gives an upper limit to the effect, love to
>see a proof that there is no rotation. Al?

Hard to do, since there really _is_ rotation, in the sense
opposite that the "forward" sprinkler turns. I saw a beautiful
realization of the forward/inverse sprinkler assembled by
Robert Berg (or somebody else associated with the demo
facility at the University of Maryland, College Park).
There is clearly a difference between forward and inverse
operation, even if, as in the Maryland demo, the sprinkler
is underwater in both modes. In forward operation, when the
flow out of the jets is suddenly halted the jets continue
to rotate for a short but noticeable time; when the same
is done in inverse operation, the jets _suddenly_ stop.

Martin Gelfand
Dept of Physics, Colorado State

[hedb...@slb.com]

In article <58qf15$p...@csugrad.cs.vt.edu>,
nur...@csugrad.cs.vt.edu (Nathan M. Urban) wrote, among other things:

> I seem to recall seeing a number of articles on the inverse sprinkler
> problem in the American Journal of Physics, though I didn't read them.
> You might want to look there.

I think that if we start out with the assumption that angular momentum must
be conserved, the direction of rotation in each case (blowing and sucking)
is clear. Experiment and math agree that rotation is in opposite
directions.
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[mfri...@ix.netcom.com]

You don't need any expensive apparatus to run this experiment. Get a
bowl of water and one of those plastic straws that has an accordionlike
region near one end so it can bend. Bend the straw about 90 degrees,
submerge the bent end in the water and hold the other loosely between
your lips. You can run the same experiment in air.