Physical quantities, Base and Derived units

All measurements taken in physics are described as physical quantities. These are seven quantities fundamental to physics. These are mass, length, time, temperature, current, amount of substance and luminous intensity.
All other quantities are derived from these base quantities. For example: speed is distance divided by time.

SI units

scientist have devised an international system that uses agreed base units for the seven base quantities. these are termed SI units.

(abbreviated SI from French: Le Système international d'unités)

Base Quantity	Unit	Symbol
Length	Meter	m
Mass	kilogram	kg
Time	second	s
Current	Ampere	A
Temperature	Kelvin	K
Amount of Substance	Mole	mol
Luminous Intensity	candela	cd

Derived units

A derived unit is a unit of measurement obtained/expressed from a base unit by making use of the mathematical symbols of division and multiplication. The International System of Units (SI) has a total of 18 derived units some of which have special names such energy is measured in joules and pressure is measured in Pascal. Lowercase letters are used in writing SI units except for those named after a person which take up and uppercase letter for the first letter.

Physical Quantity	Defining Equation	Derived unit	Base units
Speed	distance/time		m/s
Acceleration	velocity/time		m/s^2
Force	F= m x a	newton (N)	kg m s^2
Pressure	p = F/A	pascal (Pa)	kg m^-1 s^-2
Work (energy)	W = F x	joule (J)	Kg m^-2 s^-2
Power	P = W/t	watt (W)	Kg m^-2 s^-3
Charge	Q = i x t	coulomb (C)	A s
Potential difference	V = W/Q	volt (V)	kg m^-2 A^-2 s^-3
Resistance	R = V/I	ohm	kg m^-2 A^-2 s^-3

Prefixes

Many measurements are very much larger or smaller than the SI base unit. The thickness of a human hair may be a few millions of a meter and the voltage across an X-ray tube hundreds of thousands of volts.

It is often useful to write these as multiples or sub multiples of the base unit.

Prefix	Symbol	Multiple
Pico	p	10^-12
Nano	n	10^-9
Micro	u	10^-6
Milli	m	10^-3
Kilo	k	10^3
Mega	M	10^6
Giga	G	10^9
Tera	T	10^12