Dear All,
An item appeared in 'The Age' newspaper this morning that described in
some detail the anatomy of a car crash. I am sending this as an
example how the use of a metric prefix (in this case milli) can reduce
complex data in such a way that almost all values can be simplified
into whole numbers. Even people with absolutely no concept of the
length of a millisecond can get to grips with the ideas of how quickly
the crash sensors and the air bag deployment works.
Here is an extract from the article:
ALL OVER IN THE BLINK OF AN EYE
This is a reconstruction of a crash involving a stationary Ford Falcon
XT sedan being struck in the driver's door by another vehicle
travelling at 50 km/h. One millisecond equals 1/1000th of a second.
0 milliseconds - An external object touches the driver's door.
1 ms - The car's door pressure sensor detects a pressure wave.
2 ms - An acceleration sensor in the C-pillar behind the rear door
also detects a crash event.
2.5 ms - A sensor in the car's centre detects crash vibrations.
5 ms - Car's crash computer checks for insignificant crash events,
such as a shopping trolley impact or incidental contact. It is still
working out the severity of the crash. Door intrusion structure begins
to absorb energy.
6.5 ms - Door pressure sensor registers peak pressures.
7 ms - Crash computer confirms a serious crash and calculates its
actions.
8 ms - Computer sends a "fire" signal to side airbag. Meanwhile, B-
pillar begins to crumple inwards and energy begins to transfer into
cross-car load path beneath the occupant.
8.5 ms - Side airbag system fires.
15 ms - Roof begins to absorb part of the impact. Airbag bursts
through seat foam and begins to fill.
17 ms - Cross-car load path and structure under rear seat reach
maximum load. Airbag covers occupant's chest and begins to push the
shoulder away from impact zone.
20 ms - Door and B-pillar begin to push on front seat. Airbag begins
to push occupant's chest away from the impact.
27 ms - Impact velocity has halved from 50 km/h to 23.5 km/h. A
"pusher block" in the seat moves occupant's pelvis away from impact
zone. Airbag starts controlled deflation.
30 ms - The Falcon has absorbed all crash energy. Airbag remains in
place. For a brief moment, occupant experiences maximum force equal to
12 times the force of gravity.
45 ms - Occupant and airbag move together with deforming side structure.
50 ms - Crash computer unlocks car's doors. Passenger safety cell
begins to rebound, pushing doors away from occupant.
70 ms - Airbag continues to deflate. Occupant moves back towards
middle of car. Engineers classify crash as "complete".
150-300 ms - Occupant becomes aware of collision.
You will find the complete article at http://www.theage.com.au/news/news/anatomy-of-a-crash/2008/09/05/1220121518180.html
and for a short (2 page) more considered discussion of the benefits
of choosing to use whole numbers rather than fractions go to http://www.metricationmatters.com/docs/WholeNumberRule.pdf
where you will find an article that recommends a method to remove
fractions from all of your measurements and from all of your
measurement calculations. It does this by recommending that you choose
metric prefixes in such a way that fractions — both common or vulgar,
and decimal, simply become unnecessary.
Cheers,
Pat Naughtin
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
Metric system consultant, writer, and speaker, Pat Naughtin, has
helped thousands of people and hundreds of companies upgrade to the
modern metric system smoothly, quickly, and so economically that they
now save thousands each year when buying, processing, or selling for
their businesses. Pat provides services and resources for many
different trades, crafts, and professions for commercial, industrial
and government metrication leaders in Asia, Europe, and in the USA.
Pat's clients include the Australian Government, Google, NASA, NIST,
and the metric associations of Canada, the UK, and the USA. See http://www.metricationmatters.com/
for more metrication information, contact Pat at [EMAIL PROTECTED]
or to get the free 'Metrication matters' newsletter go to: http://www.metricationmatters.com/newsletter/
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