On 2007 Jul 9 , at 12:22 PM, Pat Naughtin wrote in metric matters #50:
On average, your fingernails grow at about 500 micrometres per
week; that's a millimetre in two weeks, and 2 millimetres in a month.
... and that is 26 mm in a year*.
Writing 26 mm/y is a much more reasonable way to express the result.
It is unreasonable to use such units as weeks and months to make such
measurements. The week is entirely arbitrary while the month's
relation to the phases of the moon not very significant in daily life.
The day or the year would be more reasonable if one is trying to show
how much the growth is in time periods that are significant in human
affairs and not just arbitrary. Furthermore, the day and the year are
permitted for use with SI (and have prescribed symbols to use when
used with SI units). The week and the month are not permitted for use
with SI.
Using the more meaningful and SI permitted units of days or years,
the speed of growth of a finger nail could be expressed as
70 µm/d (micrometres per day)
or
26 mm/y (millimetres per year)*.
One could also measure the speed in pure SI. The speed of 500 µm/wk
(micrometres per week) would equal:
0.8 nm/s (nanometres per second)
or
800 pm/s (picometres per second).
These expressions have the advantage of being purely SI and being
more useful in purely scientific work in cases where the relation to
human activities was not a factor.
Bill Hooper
1810 mm tall
fingernail length 10 mm
Fernandina Beach, Florida, USA
* PS
I calculated the value of 26 mm/y by dividing
the 500 micrometres per week by 7 to get micrometres per day
then multiplied by 365.25 days in a year to
get 26 000 micrometres per year
or 26 millimetres per year.
Multiplying the given value of 2 mm per month
by 12 yields 24 mm/y. However, that's not very accurate
since there are not exactly 4 weeks in a month nor are
there 12 equal months in a year.
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SImplification Begins With SI.
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