In the simplest case, the system consists of a single object acted on by a constant external force. Since it is only the object's velocity that can change, not its mass, the momentum transferred is
Δp = mΔv ,
which with the help of a = F/m and the constant-acceleration equation a = Δv/Δt becomes
Δp = maΔt
= FΔt .
Thus the rate of transfer of momentum, i.e. the number of kg·m/s absorbed per second, is simply the external force,
relationship between the force on an object and the rate of change of its momentum; valid only if the force is constant.
This is just a restatement of Newton's second law, and in fact Newton originally stated it this way.