From: craigen@ace.acadiau.ca (DOUGLAS CRAIGEN)
Subject: Re: Tapping a pop can
Organization: Acadia University
Date: Mon, 21 Dec 1992 14:34:36 GMT

In article <1h2oi5INN2os@uwm.edu> gunter@csd4.csd.uwm.edu (David O Gunter) writes:
>From: gunter@csd4.csd.uwm.edu (David O Gunter)
>Subject: Re: Tapping a pop can
>Date: 20 Dec 1992 21:28:37 GMT
>From article <BzKMK1.G7p@usenet.ucs.indiana.edu>, by mkohlhaa@silver.ucs.indiana.edu (mike):
>> Hello.  A friend and I were recently discussing the often-performed practice
>> of tapping the lid of a pop/beer can after it has been dropped, which 
>> thereby relieves the pressure within and your beverage doesn't explode.
>> 
>> Anyhow, I was pondering this and with my *limited* knowledge of physics this
>> made no sense to me.  Does anyone know if this really works, and if so, 
>> can give a physics explanation for it?
>> 
>> 					-- Mike 
>
>Well, this is just a guess, but I would think that by tapping the can you
>are causing the CO2 gas molecules dissolved within the liquid to shake free
>and settle to the top of the can.  The pressure doesn't actually decrease.
>(If you think about it, why should pressure decrease when you are agitating
>the system, thus increasing it's energy.)  Rather, when you open the can,
>the gas is closer to the opening and is released without the accompanying
>spray of liquid, since it is now separated.
>-- 
>                        | It takes a big man to cry.  But it takes a 
>david gunter            | bigger man to laugh at that man.
>gunter@csd4.csd.uwm.edu |				-J. Handy 
>gunter@mcs.anl.gov      |

   The last time I saw this discussion one of the participants claimed to 
have pressure guages in enclosed carbonated containers, and that nothing to 
do with movement of the containers (shaking, dropping, tapping) has any 
effect on the internal pressure.  In other words, the various foaming, 
or spraying effects seen are due to something besides pressure effects.

   As I recall, it wasn't even obvious that there really is such an effect.  
People will swear by a wide variety of mutually contradictory things.  For 
example, several years ago I was shown a summary of a reader poll (Ann 
Landers or something like that) on the question "how do you boil an egg so 
that the shell never sticks".  Some answers said that ever since they 
started salting the water they have never had the shell stick.  Others said 
that ever since they STOPPED salting the water they have never had a shell 
stick.  The same results came in on adding vinegar to the water, brown vs. 
white eggs, pricking the shell with a pin ......   In other words, just 
because a bunch of your friends always tap the top of their beer can, 
doesn't mean that it actually has any effect.

   As a scientist, I think you should proceed something as follows to 
establish some hard data to try explaining.  Take a couple of cases of beer 
and perform an experiment as follows:
1.  Open 3 "control" cans and note the degree of foaming.  (Better drink 
them them to avoid waste.)
2.  Shake 3 cans vigorously for 1 minute and open them.  Note the degree of 
foaming.  (Best to avoid waste again.)
3.  Shake 3 cans vigorously for 3 minutes and open them.  Note the degree of 
foaming.  (As usual concerning waste.)
4.  Shake 3 cans vigorously for 1 minute and tap once on the top.  Open them 
and note the degree of foaming.  (Remember, don't waste any.)
5.  Shake 3 cans vigorously for 1 minute and tap 3 times on the top.  Open 
them and note the degree of foaming.  (No wasting!)
    I'm sure you are a good enough scientist to fill the details for the 
rest of the experiment.  Actually, I'm starting to suspect the integrity of 
the data....

===================================================================
  Doug Craigen, Department of Physics, Acadia University,
  Wolfville, N.S., B0P 1X0, (902) 542 - 2201 x150

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Xref: sparky sci.physics:21884 sci.electronics:21736