In classical mechanics, we tend to consider objects as partcles with translational motion (or rigid bodies when considering rotational motion); that is to say that we treat them as if they possess no dimensions, and thus as if all forces act through 1 point. (the centre of mass)
Obviously using the particle model will not give us a particularly accurate answer if we are analysing a situation in which spinning or rotation occur. (eg – a ball rolling down a slope.) However, suppose we are analysing a situation in which just translational motion occurs (eg – a block of wood sliding down a slope) ; does it make any difference that the object now has dimensions? By this I mean does it matter that the forces of Friction and the Normal Reaction Force are acting upon the surface of the object, whilst the Weight still acts through the centre of mass? If not, then why, as long as the force is directed towards the centre of mass, does it not matter which part of the block the force acts on (a point inside it, or its surface) for it still be viable for it to be treated as a particle?
Thank you. Simon. :)

Definitely the weight works on the centre os mass and friction on the surface and thus the dimensions comes into picture. See you make a sheet of paper fall and it falls all zigzag but same paper rolled into a ball falls much swiftly and pridictably. I former case newtonian mechanics is not applied straight while in second you can have approx same result as with classical physics even with more accurate (and of course tedious methods). So depending upon your requirement you can use newtonian or lagrange's or hamiltonian or nay other mechanics.