If the object has some finite size, how can we analyse the motion?

Classically each and every object has some finite size, but we have assumed them as particles i.e. having mass but no size so for.

In translator motion each point on an object undergoes the same displacement as any other point as time goes on, so that the motion of one particle represent the motion of whole object.

Thus, the translator motion of an object of finite size can be studied by analyzing the motion of any constituent particle.

    But, in cases where the motion is not translator, rather than considering any point on the object, a point known as the centre of mass is defined, and the motion of this point is studied. The motion of the centre of mass leads to the analysis of the object as a whole.