2.1 Introduction
Imagine you had to make curtains and needed to buy fabric. The shop assistant would need to know how much fabric you needed. Telling her you need fabric 2 wide and 6 long would be insufficient - you have to specify the unit (i.e. 2 metres wide and 6 metres long). Without the unit the information is incomplete and the shop assistant would have to guess. If you were making curtains for a doll’s house the dimensions might be 2 centimetres wide and 6 entimetres long! It is not just lengths that have units, all physical quantities have units (e.g. time, temperature, distance, etc)

Definition: Physical Quantity
A physical quantity is anything that you can measure. For example, length, temperature, distance and time are physical quantities.

2.2 Unit Systems

2.2.1 SI Units
We will be using the SI units in this course. SI units are the internationally agreed upon units. Historically these units are based on the metric system which was developed in France at the time of the French Revolution.

Definition: SI Units
The name SI units comes from the French Syst`eme International d’Unit´es, which means international system of units.

There are seven base SI units. These are listed in Table 2.1. All physical quantities have units
which can be built from these seven base units. These seven units were defined to be the base
units. So, it is possible to create a different set of units by defining a different set of base units.
These seven units are called base units because none of them can be expressed as combinations
of the other six. This is identical to bricks and concrete being the base units of a building. You

can build different things using different combinations of bricks and concrete. The 26 letters of
the alphabet are the base units for a language like English. Many different words can be formed
by using these letters.

2.2.2 The Other Systems of Units
The SI Units are not the only units available, but they are most widely used. In Science there
are three other sets of units that can also be used. These are mentioned here for interest only.

c.g.s Units
In the c.g.s. system, the metre is replaced by the centimetre and the kilogram is replaced by the
gram. This is a simple change but it means that all units derived from these two are changed. For example, the units of force and work are different. These units are used most often in
astrophysics and atomic physics.

Imperial Units
Imperial units arose when kings and queens decided the measures that were to be used in the
land. All the imperial base units, except for the measure of time, are different to those of SI units. This is the unit system you are most likely to encounter if SI units are not used. Examples of imperial units are pounds, miles, gallons and yards. These units are used by the Americans and British. As you can imagine, having different units in use from place to place makes scientific communication very difficult. This was the motivation for adopting a set of internationally agreed upon units.

Natural Units
This is the most sophisticated choice of units. Here the most fundamental discovered quantities
(such as the speed of light) are set equal to 1. The argument for this choice is that all other quantities should be built from these fundamental units. This system of units is used in high
energy physics and quantum mechanics.

2.3 Writing Units as Words or Symbols
Unit names are always written with a lowercase first letter, for example, we write metre and litre. The symbols or abbreviations of units are also written with lowercase initials, for example m for metre and ℓ for litre. The exception to this rule is if the unit is named after a person, then the symbol is a capital letter. For example, the kelvin was named after Lord Kelvin and its symbol is K. If the abbreviation of the unit that is named after a person has two letters, the second letter is lowercase, for example Hz for hertz.

Exercise: Naming of Units
For the following symbols of units that you will come across later in this book, write whether you think the unit is named after a person or not.

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Grade 10 - Physics - Units

High School Students Studying the Sciences Physics