Units and Measurements

Lesson

Part 1 — how physics puts numbers on the world: the SI system, units for the very large and the very small, and how many digits a measurement is really worth.

In this lesson you will

Express quantities in SI units and tell base units from derived units
Use prefixes and practical units such as the light year and the angstrom
Count significant figures, round correctly and apply the rules in calculations

Every tick is a factor of 100 000 in length.

Concept

A measurement is a number times a unit

To measure a quantity is to compare it with a standard called a unit. The result is a number × unit; the number alone means nothing.

Base (fundamental) quantities are chosen independently, such as length, mass and time.
Derived quantities are built from them, such as speed, force and energy.
A system of units fixes the units of the base quantities. The international standard is the SI, built on the MKS system.

Table of the CGS, FPS, MKS and SI systems of units with the unit of length, mass and time in each
System	Length	Mass	Time
CGS	centimetre	gram	second
FPS	foot	pound	second
MKS	metre	kilogram	second
SI (international standard)	metre	kilogram	second, plus four more base units

Concept

The seven SI base units

The SI has seven base quantities and units. Since 2019 each unit is defined by fixing the value of a natural constant.

The metre is fixed through the speed of light in vacuum, $c = 299\,792\,458$ m s $^{-1}$ .
The second is fixed by the frequency of a caesium atomic clock.
The kilogram is fixed through the Planck constant.

Table of the seven SI base quantities with their units and symbols
Base quantity	SI unit	Symbol
length	metre	m
mass	kilogram	kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K
amount of substance	mole	mol
luminous intensity	candela	cd

Cookies