Re: [Phys-l] air pressure question

[brian whatcott <betwys1@sbcglobal.net>]

On 1/24/2011 3:09 PM, William Robertson wrote:
> I'm sure people are familiar with corrugated plastic tubes that make
> different sounds when you twirl them. You can hold one end above small
> bits of paper on a table and twirl the opposite end. The bits of paper
> rise up into the tube. My explanation is that by twirling the free
> end, you are accelerating the tube away from the air inside (lab frame
> of reference) or there's a centrifugal force pushing the air out
> (rotating frame of reference). That creates a low pressure area at
> that end, which would force air up into the tube. [I used to explain
> this as a Bernoulli effect, but given the discussion a while back, I
> am convinced that is incorrect]  First, is my explanation correct?
> Second, what causes the paper on the table to rise up into the tube?
> Is it just the air being pushed into the tube dragging the paper along
> with it?
>
> Bill
May I suggest you audit the chalk and talk by Ramamurti Shankar ?
I admit that I am biased by the civilized Indian gloss that Prof Shankar 
brings to it.
And you may notice that he catches himself at the too easy Bernoulli 
explanation
of airfoil lift.   If you are at all impatient, you might start at 
minute 48.
http://www.youtube.com/watch?v=lfXDJKKPGfY

About the sound tube: I can say only that I know that if you blow in 
such a tube, it will emit a tone, and blowing harder creates step 
increases in tone frequency.
I see no reason why such a tube is not acted on by low pressure at the 
whirling tip, and by some centrifugal effect as well.
Experimentally, placing a sensitive gauge or water manometer to close 
off the inner end
would provide a graph of rotation rate versus suction when the tube is 
whirled.
On the other hand, placing the outer end of the tube on an side 
extension would allow measurement of suction versus translation rate 
(i.e. when a car is driven at set speeds).
Brian W