Mechanics

The International System of Units (SI)

international system of unitsIn 1960 ,an international committee agreed on a set of definitions and standard to describe the physical quantities.The system that was established is called the international system of units (SI).

 

 

 

 

The SI system is formed from three types of units:

  • Base units
  • Derived Units
  • Supplementary units

Base units:

There are seven base units for different physical quantities, such as length, mass , time ,temperature , electric current , luminous of intensity and amount of substance.

NoPhysical quantitySymbol of quantitySI unitSymbol of unit
1LengthLmeter
2MassMkilogram
3TimeTsecond
4Electric currentIampere
5TemperatureTkelvin
6Light intensityLcandela
7Amount of substancenmole

 

Supplementary units:

The units which are neither base units nor derived units are called supplementary units and are often used for geometrical quantities. They are:

  • Plane angle
  • The solid angle

Standard definitions of supplementary units

  1. Radian

It is the angle subtended between two radii and arc of a circle whose length is equal to the radius of the circle.

  1. Steradian

It is the solid angle ( three dimensional angle) subtended at the centre of sphere by an area of surface equal to square the radius of the sphere.

Physical quantitySI unitSymbol
Plane angleRadianrad
Solid angleSteradiansr

 

Derived Units:

The SI units for measuring all other physical quantities are derived from the base and supplementary units, such units are called derived units.

Physical quantityUnitSymbolIn terms of base units
ForceNewtonNKg ms-2
WorkJouleJNm = kg m2s-2
PowerWattWJs-1
PressurepascalpaNm-2


Convection’s for indicating the Units

Use of SI units require special care , more particularly in writing prefixes:

  • Full name of the unit does not being with a capital letter even if named after scientists e.g., newton, pascal, ampere.
  • The symbol of units after a scientists has initial capital such as N for newton.
  • The prefix should be written before the unit without without any space , such as 1 mm, 1nm .
  • A combination of base units is written each with one space apart.

For example , newton meter is written as Nm.

  • Compound prefixes are not allowed.
  • When a multiple of base unit is raised to a power, the power applies to the whole multiple and not the base unit
  • Measurement in practical work should be recorded immediately in the most convenient unit e.g , micrometer screw gauges measurement in mm , and the mass of calorimeter in grams. But before calculation for the result ,all measurements must be converted to the appropriated SI base unit.

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