Constant vs Changing Mass

For constant mass

ΣF = m a

The acceleration of an object is proportional to the resultant force ΣF acting on the object.ΣF α aBy defining the newton the constant of proportionality is 1.F is the resultant force in newtons (N)
m is the mass in kg
a is the acceleration.
For changing mass

ΣF = Δ(mv)/Δt

The rate of change of momentum of an object is proportional to the resultant force ΣF acting on the object.By defining the newton the constant of proportionality is 1.F is the resultant force in newtons (N)
m is the mass in kg
a is the acceleration.