I would add one slight expansion to John Denker's good post on this subject.
He said "(If it actually mattered, the physics would tell you how to choose
which value to use in each given situation.)"
My comments are a little more aimed toward "good experimental practice"
telling you what value to use rather than the "physics" telling you, unless
you put good experimental practice under the umbrella of physics.
We have USGS data for several spots near our lab, including as close as a
quater mile away. We know the value is just above 9.80 and clearly rounds
to 9.80 rather than 9.81. I think it is a reasonably important lesson to
help students learn how to decide when using 9.81 is okay, or even 10, as
opposed to when they should use the known value for their location. John
alluded to this with his example of air pressure in Denver and how it be can
life and death in terms of air travel.
It may not be literal life and death to use a non-local number in your
research, although I suppose it could have an impact on the life/death of
you career, but I think students should learn to measure or research local
values for their lab work. If you are doing research in which your
calculations involve physical constants that are different in your lab from
some other lab, or from some world-wide average, why would you use the data
from some other lab or from a world-wide average? Doing so is introducing
some amount of error that is totally avoidable.
Using actual air pressure as opposed to standard pressure is one good
example that John mentioned. Likewise, the lab most likely is neither the
standard gas-law temperature of zero Celsius, nor is it the standard
temperature of 25 C used for many chemical reaction calculations, nor is it
the standard temperature of 20 C used for tabulated resistivity values or
for the calibration temperature of tuning forks.
Yes, using 9.80 rather than 9.81 for experimental calculations at my
location has little impact on the final numerical result. But that's not
the point. The point is making sure you realize you should use local values
for your research and therefore you should be in the habit of knowing how to
find these values and then using them. Early in fall semester when I tell
my physics students to see if they can find any published data for the value
of g for our location, they first seem surprised it is not 9.81. Then they
discover they don't have a clue how to go about finding it (other than
Google, which does not lead them to the USGS number for our location). At
some point, I suggest they go to the library and find the reference section
which has a lot of history and data about Bluffton, including physical data.
Surprise, they don't know where to go. Most of them know where the library
is, but they can't find the USGS data without help from the librarians.
It's a good exercise for the students, and the librarians love it because it
actually gets students into the library and asking questions. Of course,
once 25% of the class has done the work, the rest of the class doesn't need
the librarians anymore, and these procrastinators may not even go to the
library. Word spreads fast, even if I tell students not to tell other
students the number or where they found it.
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
1 University Drive
Bluffton, OH 45817
419.358.3270
edmiston@bluffton.edu