When an object is said to be a particle ?

Explanation

Concept of particle is most important in mechanics.
Particle means a point-like object having mass.
Particle is dimensionless, so that it is ideal concept, it is not found in real.
It can only perform linear motion.
In following cases object can be considered as particle :
(1) An object can be said to be a particle if it is moving in a straight line such that all of its particles cover same distance in same time.
(2) When the dimensions of two objects arenegligible with respect to distance between them, then those objects can be considered as particles.
Eg. : Earth and Sun are considered as particles while calculating gravitational force between them.