Physical Science:

Physics endeavors to decipher natural laws. In order to describe a natural law (or physical law) a measurement of varieties of physical quantities are essential. Measurement is a process that uses numbers and units to describe a physical quantity based on what can be observed. A physical property which can be quantified and measured in numbers and units is called a physical quantity. Length, mass, time, weight, temperature, density, speed, velocity, etc are physical quantities. Emotions, angers, feelings, appearances, etc which can not be quantized are not considered as physical quantities. In order to measure physical quantities some standard measures (or standards) must be defined on which everyone can agree. Standards are special objects that are used to make measurements in terms of fixed units of measurement. We can measure things by compering them with the standards. Units of measurement provide standards for comparisons, so that the numbers from our measurements refer to the same thing. A meter stick is an example of a standard. When something is measured with a meterstick, we compare their similar property with the meterstick. For example, if we measure a height of a pillar with a meterstick and found that height to be 6 meter then 6 is a number of times meterstick used in the measurement and meter is a unit (or length) of meterstick. This makes measurement easier and uniform.

There are three fundamental physical quantities, length, mass, and time which are independent to one another. However, to completely express our daily life physical events these three quantities would not be sufficient so some more physical quantities were included later in the list of fundamental quantity and are altogether known as the base quantities. Their units are called fundamental or base units. The units of all other physical quantities can be expressed as combinations of the base units. Such units obtained for the physical quantities are called the derived units. A complete set of these units, both the base units and derived units, is known as the system of units. The diverse human history and geo-political issues gave birth to many systems of units some of them are now obsolete and some of them are still in use at different parts of world. The three different systems of units are considered here for discussion. They are FPS (foot pound second), MKS (meter kilogram second) systems, and CGS (centimeter gram second). The FPS system of units is also called British system and MKS system is also called a metric system. The metric system is a decimalized system of measurement developed in France in late \(18^{th}\) century. Since the metric system is almost universally used, it is often referred to as the International System of Units and abbreviated SI. Since SI units use decimal system, unit conversion within the system is quite simple and convenient. The systems of units are summarized in table below.