Most physical quantities have units. Often they have more than one unit. For example the unit for distance is metre although it can also be measured in kilometres, inches, furlongs etc.

A measurement without a unit is meaningless – the length of a piece of string is always 10!

There are a standard set of units. The basic units are:

Distance: metre

Time: seconds

Mass: Kilogramme

Charge: Coulomb

It is important to use the correct units. For example if you use the formula E=mc² and the mass is not in Kilogrammes and the speed of light in metres per second you will get the wrong answer.

Unit Prefixes

You can prefix units to denote thousands, millions of the basic unit. For example a Km (Kilometer) is 1000 meters. For a full list see and a little more explanation of powers of ten see Exponents, Powers of Ten and Logs

Derived Units

Other units can be derived from those above. For example speed in measured in metres per second (m/s). The acceleration is the change in velocity per unit time and is measured in

This is sometimes written as

 

I do not expect you to remember all the following at the moment (although you will need them for basic physics). However these are just examples of how units work.

You may know that force is mass times acceleration:

Therefore the unit of force is Kilogramme metres per second squared:

i.e. the unit for mass times the unit for acceleration.

Since force is used a lot and the unit is getting a bit unwieldy this unit is often called a Newton.

Energy is Force times Distance (E = Fx). Therefore the units are newton metres (Nm). It is really

This is also called a joule (J).

Power is the amount of energy per unit time and is measured in joules per second:

This is also know as the watt (W).

Electrical charge is measured in Coulombs. A more useful unit is the amount of charge that moves through a particular point in unit times i.e. Coulombs per second (C/s). This unit is also called the Ampere (A) or more commonly amps.

The volt is also a derived unit. Since voltage times current equals power P = VI one volt is the voltage required for one amp of current to dissipate one watt of power.