# Engineering Mechanics

(Joyce) #1

(^6)  A Textbook of Engineering Mechanics
The full list of these prefixes is given in Table 1.1.
Table 1.
Factor by which the unit Standard form Prefix Abbreviation
is multiplied
1000 000 000 000 1012 Te r a T
1 000 000 000 109 giga G
1 000 000 106 mega M
1 000 103 kilo k
100 102 hecto h
10 101 deca
da
0.1 10 –1 deci d
0.01 10 –2 centi
c
0.001 10 –3 milli m
0.000 001 10 –6 micro μ
0.000 000 001 10 –9 nano n
0.000 000 000 001 10 –12 pico p
Note : These prefixes are generally becoming obsolete probably due to possible confusion.
Moreover, it is becoming a conventional practice to use only those powers of ten, which conform
to 0^3 n (where n is a positive or negative whole number).
1.18.RULES FOR S.I. UNITS
The Eleventh General Conference of Weights and Measures recommended only the fundamental
and derived units of S.I. system. But it did not elaborate the rules for the usage of these units. Later
on, many scientists and engineers held a no. of meetings for the style and usage of S.I. units. Some of
the decisions of these meetings are :

1. A dash is to be used to separate units, which are multiplied together. For example, a
newton-meter is written as N-m. It should no be confused with mN, which stands for
millinewton.

2. For numbers having 5 or more digits, the digits should be placed in groups of three separated
by spaces (instead of *commas) counting both to the left and right of the decimal point.

3. In a †four digit number, the space is not required unless the four digit number is used in
a column of numbers with 5 or more digits.
At the time of revising this book, the author sought the advice of various international authorities
regarding the use of units and their values, keeping in view the global reputation of the author as well
as his books. It was then decided to ††present the units and their values as per the recommendations
of ISO and ISI. It was decided to use :
4500 not 4 500 or 4,
7 589 000 not 7589000 or 7,589,
0.012 55 not 0.01255 or.
30 × 10^6 not 3,00,00,000 or 3 × 10^7

``````* In certain countries, comma is still used as the decimal marker.
† In certain countries, space is used even in a four digit number.
†† In some question papers, standard values are not used. The author has tried to avoid such questions in
the text of the book, in order to avoid possible confusion. But at certain places, such questions have been
included keeping in view the importance of question from the reader’s angle.``````