Kinematics of A Particle : Along A Straight Path:

We can define a particle as a material point without dimensions, but having a definite quantity of matter. In kinematics we are concerned only with the time displacement relationship of a given motion of a particle or a rigid body.

When the size of a particle is extremely small compared with its range of motion, in certain cases we can consider it as a particle. Stars and planets, although quite large in diameter by themselves, can be considered as very small compared with their range of motion and may be considered as particles in a very large space. Similarly, the dimensions of a bullet or a rifle are very small compared with those of its trajectories, and thus the bullet may be considered as a particle.

When a particle moves, it describes a curve that is called a path. If the path described by a particle is straight line, the particle is said to have rectilinear motion. The displacement is a vector quantity and therefore, all vector methods are applicable to the displacement, velocity & acceleration relationships.