Length, Mass and Time
In general, the three observable Physical quantities Length Mass and Time
that can be measured directly are named as the fundamental physical quantities
through which others physical quantities can be derived. All measurements are
necessarily approximations, a great deal of effort must be taken to make
measurements as accurate as possible.
Units for Measurement
Einstein (1933) has said, "Pure logical thinking cannot yield us any knowledge of the empirical world; all knowledge starts from experience and ends in it. Propositions arrived at by purely logical means are completely empty as regards reality". Standard measurements as part of experimentation is essential part of research as well as application of knowledge.
When you go to buy sugar, you do not say that "I want to buy a sugar", you will say "I want to buy 1kg of sugar". The shopkeeper then weighs the sugar, against STANDARD weights and gives you 1 kg of sugar. Similarly, when you measure with a ruler, say length of a pencil, the markings on the ruler are STANDARD lengths. Take another physical quantity, say time. A second, a minute and an hour on your watch has been set to a STANDARD. Now you may ask, why this is so? The answer to this question is that anywhere on the earth, your measurements should match anybody else's measurements.
When you say that you have bought 10 kg of potatoes, whether you buy them in Pune, Paris or Portsmouth, 10 kg of potatoes have to be 10 kg of potatoes and nothing else. Therefore, whenever you measure any physical quantity, they are relative to or compared against some STANDARD measurements. The international community from time to time announces new, revised standards and all nations have to comply with these standards.
The basic quantity of the standard is known as a unit. The amount of a physical quantity that is used as a reference for the measurement of that quantity is called the unit of that quantity. The unit is invariable, easily reproducible and is internationally accepted while an actual measurement would be a fixed multiple of the unit. The multiple is called the magnitude of the unit. For example, if we decide unit length to be 1 meter, and if a length of the side of the table is 2.0 meters then magnitude of the measurement is 2.0.