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CHAPTERS BY WACEK KIJEWSKI

# CHAPTER V:  The Characteristics of the SI System

1. The SI system is a decimal system with each component a multiple or submultiples of 10. All calculations and conversions are therefore simplified.

E.g. 1 km = 10³ m = 10⁵ cm = 10⁶ mm 

2. Decimal multiples and submultiples can be attached to any unit if it is necessary to make a unit of a more convenient size. These multiples and submultiples have their names in the form of prefixes (Table 1). The prefixes with powers which are multiples of three are preferred. Only one prefix is used with a unit at a time.

E.g. 10⁶ millimetres = 10³ metres = 1 kilometre
0.007 5 metres = 7.5 millimetres
10^-9seconds = 10^-6 milliseconds = 10^-3 microseconds = 1 nanosecond
“nanometre” but not “millimicrometre”

3. The SI units are coherent, i.e. there is only one unit for each quantity, and units of different quantities are combined without conversion factors. Substituting basic or derived units for quantities in a formula ensures that the quantity representing the answer is expressed in a correct basic or derived SI unit.

E.g. 1: A body of a mass 500 g is raised to a height of 24 cm above the ground. Find work done against the force of gravity. 
(Assume g = 10 m s^-2 .)

F (N) = m (kg) × g (m s^-2 )
W (J) = F (N) × s (m)

F = 0.500 kg × 10 m s^-2 = 5.0 N 
W = 5.0 N × 0.24 m = 1.2 J

The answer: W = 1.2 J

Do you like anecdotes, interesting and challenging problems, fun facts, puzzles, jokes related to metric system and measurement? Read them in the 2006 on-line edition of "SI Units, Conversion and Measurement Skills",186 pp.

Wacek Kijewski
Metric Fun
"SI Units, Conversion and Measurement Skills"
PO Box 2358 George 6530 South Africa
tel/fax: +27 44 884 0486

About the Author

Wacek Kijewski: born in Poland, studied electronics, telecommunication, acoustics and pedagogy. He taught science in Poland, Hungary and became Lecturer and Principal Tutor in Nigeria and Botswana.
Mr. D.D. Kuhlmann, Ag Registrar, University of Botswana (UB): “Dr W. Kijewski has performed his duties very satisfactorily and proved that he is an excellent teacher with substantial relevant experience. He has an admirable character which coupled with his professional integrity enabled him to established an excellent rapport with students.”

Dr. L.S. Molema, English Dept., UB: “Students speak warmly of him, especially his capacity for making them understand and feel confident in a subject that they had previously been inadequately prepared for and were therefore scared of. I have just finished editing the proofs of his new book: “S I Units, Conversion and Measurement Skills” in which his teaching style, so popular with his students, is evident.”

Donate to support this site

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