# Units part I

Four basic metric instruments .

Let’s say that we finally solved a problem in physics and the answer is 54 . Number 54 doesn’t really answer our problem , it can represent minutes , Newtons , Joules , Kelvin degrees and anything that you can imagine (such as smartphones , tablets , and so on) .

For examble , we are trying to calculate the distance that a small ant walked form point A to point B in just 2 seconds . The answer is AB = 20 , if number 20 is mm (millimeters) or 2 cm that is a possible answer , if number 20 is cm (centimeters) the ant is running and finally if number 20 is dm (decimeters) we have an impossible answer .

Physical Quantity is anything that can be measured . We live in a world with plenty of physical quantites such as distance , time , speed , acceleration , force , energy and many more .

There are many unit systems in use today. Physicists use four main sets of units: SI units, c.g.s units, imperial units and natural units .

SI Units

SI means international system of units , there are seven base SI units .

These seven units are called base units because none of them can be expressed as combinations of the other six.

Base quantites are :

• length , meter (m) ,
• mass , kilogram (kg) ,
• time , second (s) ,
• electric current , ampere (A) ,
• temperature , kelvin (K) ,
• amount of substance , mol (n) ,
• luminous intensity , candela (cd) .

All physical quantities have units which can be built from these seven base units.

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.

Imperial Units

Examples of imperial units are pounds, miles, gallons and yards. These units are used by the Americans and British.

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.

We notice that different systems of units exist, we must be able to convert between systems of units .