INTRODUCTION:
“MEASUREMENT” is the determination of the size or magnitude of something. Measurement can also be defined as “Comparison of an unknown quantity with some known quantity of the same kind”.
Measurement of an object consists of:
(1) The unit of measurement.
(2) The numerical value (or) Magnitude
Unit of Measurement
A unit of measurement is a definite magnitude of a quantity, defined and adopted by convention or by law, that is used as a standard for measurement of the same kind of quantity. Any other quantity of that kind can be expressed as a multiple of the unit of measurement. For example, a length is a physical quantity.
PHYSICAL QUANTITIES:
A quantity which can be measured and by which various physical happenings can be explained and expressed in form of laws is called a physical quantity. For example, length, mass, time, force etc.
On the other hand, various happenings in life e.g., happiness, sorrow etc. are not physical quantities because these cannot be measured.
Measurement is necessary to determine magnitude of a physical quantity, to compare two similar physical quantities and to prove physical laws or equations.
A physical quantity is represented completely by its magnitude and unit. For example, 10 meter means a length which is ten times the unit of length 1m. Here 10 represents the numerical value of the given quantity and meter represents the unit of quantity under consideration. Thus in expressing a physical quantity we choose a unit and then find that how many times that unit is contained in the given physical quantity, i.e.
Physical quantity (Q) = Magnitude × Unit =n × u
\(n\alpha \frac{1}{u}\)
\(n_1u_1=n_2u_2\)
Where, n represents the numerical value and u represents the unit. Thus while expressing definite amount of physical quantity, it is clear that as the unit(u) changes, the magnitude(n) will also change but product ‘nu’ will remain same.
Examples: Mass, amount of substance, length, time, temperature, electric current, light intensity, force, velocity, density etc.
TYPES OF PHYSICAL QUANTITIES:
Any quantity which can be measured directly (or) indirectly (or) in terms of which any laws of physics can be expressed is called physical quantity
There are two types of physical quantities
1) Fundamental quantities
2) Derived quantities
Fundamental Quantity: Physical quantities which cannot be expressed in terms of any other physical quantities are called fundamental physical quantities.
E.g . length, mass, time, temperature etc..
Derived Quantity: Physical Quantities which are derived from fundamental quantities are called derived quantities.
E.g. Area, density, force etc..
UNITS & TYPES OF UNITS
Normally each physical quantity requires a unit or standard for its specification so it appears that there must be as many units as there are physical quantities.So any unit of mass, length and time in mechanics is called a fundamental, absolute or base unit. Other units which can be expressed in terms of fundamental units, are called derived units. For example light year or km is a fundamental units as it is a unit of length while \(s^{-1}\), \(m^2\) or kg/m are derived units as these are derived from units of time, mass and length respectively.
Systems of Units :
There are four systems of units
1. F.P.S
2. C.G.S
3. M.K.S
4. SI
A unit system is composed of the variants upon one standard scientific unit of measurement.
The common systems are given below:
(1) FPS system : In this system foot, pound and second are used respectively for measurements of length, mass and time. In this system force is a derived quantity with unit poundal.
(2) CGS system : The system is also called Gaussian system of units. In it length, mass and time have been chosen as the fundamental quantities and corresponding fundamental units are centimeter (cm), gram (g) and second (s) respectively.
(3) MKS system : The system is also called Giorgi system. In this system also length, mass and time have been taken as fundamental quantities, and the corresponding fundamental units are meter, kilogram and second.
(4) S. I. system : It is known as International system of units, and is infact extended system of units applied to whole physics. There are seven fundamental quantities in this system.