The metaphor of momentum is essential to the development and exploration of RDT. The metaphor is based on Newton’s second law of motion:, which states that when an external force is applied to an object in motion, the change in velocity is related directly to the magnitude of the force and is related inversely to the object’s inertial mass. The metanymical extension of the concept and assigned term in behavior analysis suggests that when an external force (a "disrupter") is applied to ongoing behavior, kiany decrease in response rate is directly related to the magnitude of the disrupter, and is related inversely to the behavioral equivalent of mass. Just as inertial mass is independent of the velocity of a physical body in motion, behavioral mass is independent of response rate, both conceptually and empirically. This all sounds logical and systematic as a derivation of state, but the italized predicate lacks context.
That context is that mass times velocity equals momentum (MV=P), where V is measured as a constant rate/unit of time (e.g., 60 miles per hour, or 60 responses per minute). This is an important detail that should be maintained within any discussion or characterization of behavioral momentum, as P ensures symmetry in the relation between initial and kinetic energy, or behaviorally, rate of responding, or log(Bx/Bo) (a behavioral interpretation of the "Lagrangian" in physics). Energy is varied in terms of the speed of repition of an event (i.e., its "frequency"). The rate at which a pellet passes a particular point in succession to its predecessor directly informs you of the energy of the system you are dealing with. As that energy level increases the rate of each pellet passing that particular point increases. Now imagine that each pellet is a behavioral event within a response cycle (i.e., a repetitive behavioral event with a distinct beginning and end that sets the "cycle" for that event. The main point is, that like the Lagrangian tracks the change in the kinetic and potential energy of a system. Or, behaviorally, delta Bx, the observed change in response rate, is expressed as the difference between asymptotic response rate, Bxa, and response rate during disruptions, Bxd = Bx = Bxa-Bxd, eventually expressed as log(Bxa/Bxd). The upshot of this analysis is that Rate of Responding is critical to the conduct of research on RDT as Asymptotic Maximum rates of responding set the upper bondary state of the system, beyond which further evolution requires the emergence of otherwise unique structures and functions that form their won "density" and "momentum".