In this video sir shahzad will explain about physical quantities and international system of units The foundation of physics rests upon physical quantities in terms of which the laws of physics are expressed. Therefore, these quantities have to be measured accurately. Among these are mass, length, time, velocity, force, density, temperature, electric current, and numerous others.
Physical quantities are often divided into two categories: base quantities and derived quantities. Derived quantities are those whose definitions are based on other physical quantities. Velocity, acceleration, and force etc. are usually viewed as derived quantities. Base quantities are not defined in terms of other physical quantities. The base quantities are the minimum number of those physical quantities in terms of other physical quantities can be defined. Typical examples of base quantities are length, mass and time.
The measurement of a base quantity involves two steps: first, the choice of standard, and second, the establishment of a procedure for comparing the quantity to be measured with the standard so that a number and a unit are determined as the measure of that quantity.
An ideal standard has two principle characteristics: it is accessible and invariable. These two requirements are often incompatible and a compromise has to be made between them.
In 1960, an international committee agreed on a set of definitions and standard to describe the physical quantities. The system that was established is called the System International (SI).
Due to simplicity and convenience with the units in this system are amenable to arithmetical manipulation, it is in universal use by the world's scientific community and by most nations. The system international (SI) is built up from three kinds of units : base units, supplementary units and derived units.
There are seven base units for various physical quantities namely: length, mass, time, temperature, electric current, luminous intensity, and amount of substance ( with special reference to the number of particles).
The names of base units are metre, kilogram, second, ampere, kelvin, candela, mole.
The general conference on weights and measures has not yet classified certain units of the SI under either base units or derived units. These SI units are called Supplementary Units. For the time being this class contains only two units of purely geometrical quantities, which are plane angle and the solid angle.
The radian is the plane angle between two radii of a circle which cut off on the circumference an arc, equal in length to the radius.
The steradian is the solid angle (three dimensional angle) subtended at the center of a sphere by an area of its surface equal to the square of radius of the sphere.
SI units for measuring all other physical quantities are derived from base and supplementary units.
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