Certain quantities are basic in the study of mechanics and they should be studied at the beginning.
They are stated below.
（i) Matter. It may be defined as that which occupies space and possesses weight. It can be
perceived by our senses.
（ii) Body. It may be defined as a limited portion of matter and composed of very large
a number of small particles.
（iii) Particle. A body of negligible dimensions is called a particle. In the mathematical sense a
particle is a body whose dimensions approach zero.
The quantities which exist independently i.e., cannot be expressed in terms of one another are
called Basic or Fundamental quantities of mechanics.
1. Length: The basic quantity used for the measurement of the distance between points in
space is called length. The length of an object is expressed by comparing it with a standard
length. It is denoted by ‘L’.
2. Mass: The amount of matter contained in a body is called it’s mass. It is the measure of
the inertia of a body. It is constant for all cases in mechanics.
The mass of the body is denoted by ‘M’ and expresses its magnitude.
The unit of mass in the SI system is a kilogram which is the mass of 5·0188 x 1025 atoms of
carbon-12.
3. Time: The basic quantity used for making the flow of events is called time. It is denoted
by ‘T’.
（iv) Distance. The total length of the actual path covered by a body is called as distance
traversed by the body. It is a scalar quantity and its unit in the SI system is metre. One metre is the
distance that contains 650763·73 wavelengths of orange-red light of Kr-86.