Hello. Let's image a bar. In one side is attached to a body so the bar can rotate over this axis. There is no friction between the two bodies. The system is at rest in t=0. A force acts forming a 90° angle with the bar. The bar moves and begin to rotates. The force dissapear. There is no force acting on this system, but I think that the bar doesn´t keep moving forever since its changing his direction every moment, hence there is an acceleration present, but if there is an acceleration present a force should be acting in the system which contradicts the assumption. ¿How this can be?