The kinematics and the concept of a point particle
Kinematics in Greek kinema-atos It means movement. It describes the motion of objects without concerning the causes which determine.
The bodies of which studies the bikes are varied: a truck that moves along a road, a ship that moves on ’ water, an electron revolving around the nucleus of an atom or the earth revolves around the Sun. You will then need to write different equations of motion for a missile or a ball? To overcome this first question you have to introduce the concept of point particle (motion of bodies that can be localized by specifying the position of a geometrical point with mass).
It's called point particle a geometrical point with mass.
It is therefore appropriate to treat a body as point particle when its size is negligible compared to the phenomenon in the studio and do not consider the rotation.