Isaac Newton developed his Second Law of Motion by examining the change in an object's momentum over time when a net force is applied. What he presented was:
F = (Pf - Pi)/t where Pf and Pi are the final and initial momentum respectively.
P = mv and in Newton's initial analysis he kept the mass constant producing
F = (mVf - mVi)/t = m(Vf - Vi)/t
By definition, (Vf - Vi)/t is acceleration, so
F = ma. Solving this for a gives
a = F/m
Therefore, the ratio of the net force on an object and its mass is the resulting acceleration of the object.