While deriving formulas/describing motion on an inclined plane, why don't we take into account the shape of the object undergoing motion? e.g. 1. If I take an inclined plane and place a ball on it, only a small amount of inclination would make the ball move down the plane... But... 2. If a take a matchbox and place it on the inclined plane, a lot of inclination would make it move down... Why aren't such factors taken into account?