# The International System of Units

People from various countries use three different systems of units for measurements. These systems have been listed below, along with their base units for length, mass, and time:

- The
**CGS system**(centimeter, gram, and second) - The
**MKS system**(meter, kilogram, and second) - The
**FPS system**(foot, pound, and second)

But technically, the current internationally accepted system of units for measurement is the **Systeme Internationale d’ Unites**, abbreviated as SI and generally called the International System of Units.

SI units involve the decimal system; thus, conversions within this system are pretty convenient and relatively simple.

## Units: Fundamental and Derived

The units for the base or fundamental quantities are known as **base **or **fundamental units**. The units of all other physical quantities, which can be expressed as combinations of the fundamental units, are known as **derived units**.

A complete set of the fundamental and derived units is called the **system of units**.

Given below are the four basic properties of units:

- They should be readily available and reproducible.
- They need to be well-defined.
- They should be invariable.
- They should be acceptable to everyone.

### Fundamental Units

There are **seven fundamental or base units** in the SI system, which are completely independent of each other and one can express all other physical quantities in terms of these fundamental units. They have been listed in the table given below:

Apart from these seven base units, there are two more units that are defined for:

**Plane angle**dθ as the ratio of length of arc ds to the radius r**Solid angle**dΩ as the ratio of the intercepted area dA of the spherical surface, described about the apex O as the center, to the square of its radius r.

The unit for plane angle is **radian **(symbol = rad), while the unit for the solid angle is **steradian **(symbol = sr). Both of them are dimensionless quantities.

Whenever “mole” is used, the elementary entities in question (atoms, molecules, electrons, ions, etc.) must be clearly specified.

### Derived Units

The units of all other physical quantities, which can be expressed as combinations of the fundamental units, are known as **derived units**. These derived physical quantities, such as speed, can be expressed in terms of fundamental physical quantities.