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 10^{25} 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.