> Well the easiest way is on key pressing down, do the spin animation, when the key is released, keep the animation but move sonic forward at a much faster pace.
> Is that what you mean? I'm not totally sure what you mean by physics of motion.
as changing the angle of the sprite when it rotates 360 degrees for the platform?
This isn't a trivial matter as changing the angle of the sprite is the algorithm that gives Sonic games their novelty and is difficult to replicate.
This gives some details on the algorithm. I also recommend looking at Sonic Physics Guide, which is guide designed for creating Sonic fan games that gives details about the algorithms and variables used to create the classic games.