Re: Bernoulli and viscosity

[Wolfgang Rueckner <rueckner@FAS.HARVARD.EDU>]

Your suggestions below (except for the moon test) are actually
standard demos we perform for classes.

(1)  A tall bucket full of water has two openings at the bottom with
standard plumbing elbows inserted into the openings so that the water
squirts up.  One of the two elbows is "pinched off" so that the
opening is very small.  The students are asked to predict which one
squirts higher.  They predict the pinched off opening.  According to
Torricelli's law (derived from Bernoulli's), the velocity of efflux
should be the same and, indeed, they DO squirt to the same height.
They should also squirt as high as the water level in the bucket, but
not quite because of frictional losses.

(2)  Punch a small hole in the side of a plastic cup near the bottom.
Fill it with water and of course it squirts out.  Drop the cup from
an appreciable height (so students have time to observe -- use a
ladder) into a waste basket and show that the velocity of efflux is
zero -- no water comes out of the hole.  g is zero in the reference
frame of the falling cup.

(3)  I later follow up on (1) above when discussing Poiseuille's law.
The experimental result of (1) is quite counterintuitive to the
students because they have experience in pinching off a hose and
having the water squirt higher.  Thus another bucket full of water
has two small "hoses" (small diameter rubber tubing) coming out of
the bottom.  The hoses are of equal length and I.D.  Both squirt
water to the same height when aimed upward.  But if the end of one is
pinched to make the opening smaller, it squirts much higher (more
than a factor of 10).

Wolfgang



> This is another result which is (contra-intuitively?) independent of the
> value of g, but is clearly invalid for g = 0.  In that case the derivation
> encounters 0/0.
>
> A moon test would be interesting.
> How about a test on a rotating platform?
> Good demo: a free fall test.
>
> Bob Sciamanda (W3NLV)
> Physics, Edinboro Univ of PA (em)
> trebor@velocity.net
> http://www.velocity.net/~trebor
> ----- Original Message -----
> From: "John Mallinckrodt" <ajm@CSUPOMONA.EDU>
> To: <PHYS-L@lists.nau.edu>
> Sent: Wednesday, December 11, 2002 6:30 PM
> Subject: Re: Bernoulli and viscosity
> | . . .
> | That analysis predicts a range of 2[h*H]^1/2 where h is the depth of
> | the hole and H is its height above the surface on to which the stream
> | plays . . .