Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: [Phys-L] Why **UNITS** matter!



Greetings. I always tell my students, "let the units do the talking." If the ending unit(s) are appropriate with the unknown desired, then the probability is that you have done things correctly.

If students work problems without using the units, I will not give credit for the solution as it is incomplete.

Have a Great Day

Website rheckathorn.weebly.com



On Aug 10, 2016, at 2:50 PM, rjensen@ualberta.ca wrote:

The Gimli Glider and Mars Climate Orbiter are common examples of why
it is important to include *units* in calculations.

An example from 2015/16 is documented in the article below.
http://cen.acs.org/articles/94/web/2016/08/Fracking-air-pollution-study-retracted.html

So they used the wrong value of R. This is a *first year* mistake!
But it goes further than that. The revised article states,
"Honest calculation errors resulted from using incorrect units of the
ideal gas constant and improper cell linkages in the spreadsheet ...."

*Improper cell linkages* This happens, and is often difficult to
identify these errors.

This article and these errors could be used to drive home several
messages:
1. units must be included in all calculations
2. you must do a few calculations by hand (with units) to confirm your
spreadsheet formula is correct
3. <in group projects> having several people independently complete
calculations will find and correct errors

Failure to do this could be catastrophic to your career and your
employer. In this particular case, litigation by petroleum companies
to recover lost revenue is possible.

It is not always possible to judge the reasonableness of the final
answer. In this particular case, while the erroneous answer was 300
times greater than the correctly calculated value, it is still
reasonable from a chemistry perspective. Granted, the authors were
expected a value higher than ambient, so didn't question an answer
that was. The corrected values are significantly lower than every
other reported site (Figure 2 in the corrected paper). So personally,
I don't trust the corrected values either.

Roy Jensen

_______________________________________________
Forum for Physics Educators
Phys-l@www.phys-l.org
http://www.phys-l.org/mailman/listinfo/phys-l