  A transition rate model of cargo transport by $N$ molecular motors is
proposed. Under the assumption of steady state, the force-velocity curve of
multi-motor system can be derived from the force-velocity curve of single
motor. Our work shows, in the case of low load, the velocity of multi-motor
system can decrease or increase with increasing motor number, which is
dependent on the single motor force-velocity curve. And most commonly, the
velocity decreases. This gives a possible explanation to some recent
