Fractions in the SI System?
Date: 11/07/2001 at 16:21:02 From: steve gissendanner Subject: Proper metric terminology When using metric, can 4 cm and 5 mm be written as 4 1/2 mm? I think 4.5 cm would be the only correct terminology. The question arises from a standardized test question. Thanks, Giss
Date: 11/08/2001 at 12:03:09 From: Doctor Peterson Subject: Re: proper metric terminology Hi, Steve. It's my understanding, too, that fractions are disparaged in the metric system; but most of the metric style guides I know of don't seem to mention them - though they don't give any examples with fractions, either. For your reference, here are some of them: NIST Reference on Constants, Units, and Uncertainty http://physics.nist.gov/cuu/Units/index.html NIST Guide for the Use of the International System of Units (SI) http://physics.nist.gov/Pubs/SP811/sp811.html Correct SI* metric system usage - U.S. Metric Association), Inc. http://lamar.colostate.edu/~hillger/correct.htm The Metric System (SI)- A Concise Reference Guide - David Bartlett http://www.hlalapansi.demon.co.uk/Metric/index.html But I ran into two that do explicitly say not to use fractions: Metrics the Right Way http://www.pnl.gov/ag/usage/metrics.html Mixed units (or mixed multiples), which are commonly used in the inch-pound system, are not used in metric practice. For example, you may write an inch-pound length as 5 ft, 8 in., but write the metric length as 1.73 m, not 1 m, 73 cm. Binary fractions (such as 1/2 or 3/8) are not used with metric units. For example, you would give a person's weight as 70.5 kg, not 70-1/2 kg. When spelling out numerical values, decimal fractions are acceptable. For example, 0.4 A would be spelled out as four-tenths ampere. By "binary fractions" they apparently mean "common fractions," which is what we are talking about. Again, The International System of Units (SI) by Dennis Brownridge http://lamar.colostate.edu/~hillger/brownridge.html#correct-use Use decimals, not fractions. Do not use more than one prefix with a unit, or a mixture of units. Correct Incorrect 8.5 kg 8 1/2 kg 3.15 m 3 m 15 cm So "officially," fractions are not used in the SI system; but in more informal circumstances, particularly with children, I wouldn't personally make strong objections. You might be interested in this page on household use of metric, which starts off with a fractional metric measurement: METRIC LAND - Joan Pontius http://www.rci.rutgers.edu/~jup/metric/metric_land.html - Doctor Peterson, The Math Forum http://mathforum.org/dr.math/
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