Re: [Phys-l] 9.80 vs. 9.81

[John Denker <jsd@av8n.com>]

On 04/19/2008 06:16 PM, Larry Smith wrote:
> The value of g at my location is less than 9.80 m/s^2, and Randy Knight
> uses 9.80 in his textbook, but NIST says the accepted value for standard
> gravity is 9.80665 which rounds to 9.81.  Most textbooks use 9.81 when they
> want three sig-figs (but not Knight).
>
> Any thoughts?

As far as I'm concerned, the relevant rule here is:
  Say what you mean, and mean what you say.

That is:
  a) If you want the standard acceleration of gravity, just say so.
  b) If you want the actual local acceleration of gravity at
   such-and-such time and place, just say so.  


A more extreme application of the same principle concerns atmospheric
pressure.  The "standard atmosphere" is defined as 1 atm := 101325 Pa 
exactly:
 http://physics.nist.gov/Pubs/SP811/sec05.html

However, the ambient pressure in (say) Denver has never been equal to 
1 atm.  Not even close.  
 -- The difference has a significant effect on the cooking time for
  hard-boiled eggs.  
 -- It is literally a matter of life and death when calculating aircraft 
  takeoff and landing performance.  


Henry Kissinger said that academic fights are particularly vicious
because there is so little to fight about.  Jonathan Swift also had
something to say about this.
  http://www.online-literature.com/swift/gulliver/5/
AFAICT the main reason students get worked up about the distinction 
between g and g_n is that for typical introductory classroom applications 
it doesn't matter.  (If it actually mattered, the physics would tell 
you how to choose which value to use in each given situation.)