Re: reverse water sprinkler

[brian whatcott <inet@INTELLISYS.NET>]

At 16:08 12/7/99 -0500, David Strasburger wrote:
> // a question about a water sprinkler and
> which way would it spin if you ran it backwards. ///
> A popular kind of lawn sprinkler has a head which is free to rotate and
> arms that are crooked in the horizontal plane. ///
>  I thought about it for a while,
> mostly in terms of momentum or Newton's 3rd law. ///
>  Set to suck, the armature rotates in the opposite direction.
> Very decisively. /// Surely I am missing something
> obvious!
>
>
> David Strasburger

I expect I'll regret this - I can't remember Feynman's conclusion -
I thought it was non -intuitive.

Here's how I rationalize the cranked arm sucking water, submerged.
 The radial and axial flow I ignore. In the cranked arm, if held
still, there is a tangential inlet water column mass accelerated
from still water.

 This provides a tangential force, which if allowed to rotate the
arm, reduces the acceleration of the inlet water.
The arm's rotation is also opposed by the frictional drag of the
circumambient fluid. There is a torque balance at some speed,
while water and sprinkler arm head in opposite directions.

 It is not necessary to have a mental model of how a
change of momentum is associated with a force - you know that it
always is.
 This next step is the non intuitive aspect of hydrodynamics
that causes the problems, I imagine.





brian whatcott <inet@intellisys.net>
Altus OK