Now for a particle in a time-independent potential, specifying position and velocity at a given instant determines the future path—but that is not the case here, the acceleration is determined by the ...Now for a particle in a time-independent potential, specifying position and velocity at a given instant determines the future path—but that is not the case here, the acceleration is determined by the phase of the driving force, which is time varying, so the system really needs three parameters to specify its subsequent motion.
