I find difficulties in identify the forces acting behind the acceleration of objects that are considered consequence of conservation principles (for istance of KE and angular momentum). I'll make an example to explain. The same string-mass system is linked to a rod. In case (a) a force pull the string while the mass is spinning, in case (b) the string goes around the rod as the mass spins. In both cases the mass will accelerate but I'm trying to understand what are the forces behind this acceleration. I'm ok with case (a): the force is central and so angular momentum is conserved. The force is not completely radial and the component of the force parallel to displacement change the velocity (in fact kinetic energy is not conserved) But in case (b) the force is not central and angular momentum is not conserved, while energy is because force is always parallel to the displacement. From the conservation of KE we find that the velocity increases. But which is the force responsible for that? I mean: there must be a force acting in the direction of the displacement if the velocity increases. Where is this force?