In a variable mass constant velocity system, how can we calculate force?

In its most general form, Newton's second law states that the rate of change of a particle's momentum p is supplied by the force acting on the particle, i.e., F=d p / d t.
If there is no force acting on the particle, then $\mathrm{p}$ must be constant, or conserved, because $d p / d t=0 .$
The general expression for force is F=d p / d t.
On solving we get, F=m(d v / d t)+v(d m / d t).
Since the system is constant velocity, hence d v / d t=0. What remains is F=v(d m / d t).