# CHAPTERS BY WACEK KIJEWSKI

# # Chapter 05 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 = 10^{3} m = 10^{5} cm = 10^{6} 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. 10^{6} millimetres = 10^{3} metres = 1 kilometre

0.007 5 metres = 7.5 millimetres

10^{-9}seconds = 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.