[ExI] underwater sprinkler, was: RE: Musical instruments in space

[spike]

From: extropy-chat-bounces at lists.extropy.org
[mailto:extropy-chat-bounces at lists.extropy.org] On Behalf Of Dave Sill
Sent: Saturday, March 30, 2013 10:53 AM
To: ExI chat list
Subject: Re: [ExI] underwater sprinkler, was: RE: Musical instruments in
space

 

On Fri, Mar 8, 2013 at 9:26 AM, spike <spike at rainier66.com> wrote:

An example of the kinds of stuff we used to do is from a Feynman question.
You have seen those S-shaped lawn sprinklers, turn on the water, they spin.
What would happen if you took one of those, submerged it in water, connected
the hose to a pump and pulled water thru it backwards?  Would it turn the
opposite direction, since the tips of the arms would act as nozzles in
reverse?  Or would it turn the same direction as before, since the water is
creating centrifugal force as it goes around the bend?  Or would the forces
exactly cancel and not turn?  Or a fourth possibility that blows your mind?

I know the answer from experiment.  Can you figure it out from math and
thought experiment?


So what's the answer?

 

-Dave

 

 

Dave, this one still blows my mind.  As I think John Clark pointed out,
conservation of momentum requires that there is no net rotation.  Otherwise
you would be creating angular momentum out of nothing.  However, the concept
is often misunderstood, and John missed something important.

 

You have perhaps heard that if you hold a cat by her feet and release, the
cat will land on her feet.  This seems to violate the concept of
conservation of angular momentum, but it doesn't.  Cats are clever in that
way, but not magic.  They borrow some angular momentum, get feet downward,
then pay back.  You can google on how cats do this, or check out the video
and move to about 1:15 in this excellent presentation:

 

http://www.huffingtonpost.com/2012/08/24/why-cats-always-land-on-their-feet-
_n_1828748.html

 

Regarding the underwater sprinkler, during the time in which the flow rate
is increasing, there is a counter-rotation.  During the time in which the
flow rate is decreasing, the borrowed angular momentum must be paid back,
exactly with no interest.  So during that phase, there is a rotation in the
same direction as the above-water sprinkler.

 

Some of you mathematical hotties, do explain that observation please, using
differential equations, or whatever is your favorite mathematical
technology, including even a digital model or a Matlab sim.  If you manage
it, the grand prize will be yours: my sincere everlasting admiration.

 

spike

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