Units

Units

• The SI system of units will be followed throughout this book. In this system, the basic units are meter (m) for length, kilogram (kg) for mass and second (s) for time. Units of other quantities are written in terms of these basic units.

• For example, units of velocity are m/s, units of acceleration are m/s2, units of momentum are kg-m/s2, etc. These are called derived units.

• Some of these derived units have been given special names. For example, unit of force is Newton (N) which represents kg-m/s2. A summary of the derived units required in mechanics is given in Table 1.11.1.

• In addition to the base units, the unit for plane angle - radian (rad) is used as a supplementary S.I. unit.

• For multiples and submultiples of the basic SI units, prefixes like 'kilo' and 'milli' are used. These are summarised in Table 1.11.2. The commonly used prefixes are shown in bold type.

• For Example,

1 km = 1000m

1 mm = 10-3m

1 kg = 103g

• In addition to the above mentioned units, the minute (min), hour (h), day and year are permitted in SI system for time.

1 min = 60 s

1 h = 60 min = 3600 s

• S.I. system also permits the use of degree (°), minute ('), second (") and revolution (r) as units for plane angle.