What is meant by a particle, a system and internal and external forces?

A particle is an idealized small-sized object with mass but negligible in size, and used in physics to simplify the problems by taking the objects as point masses. The system means a collection of particles or objects that are taken for analysis, and it may be isolated from the external influences or open to them. The forces exerted by the particles inside a system on each other do not affect the total momentum of the system, hence termed internal forces. In contrast, external forces act on the system from outside its boundaries, changing its momentum and influencing its overall motion. Understanding these concepts is essential for analyzing physical interactions and dynamics.