why is spring force not consided as impulsive force? it also adjusts accordingly
An impulse by definition acts instantaneously (almost zero elapsed time). As a result a system under an impulse will have its momentum changed during this very short period of time. The forces acting during the impulse are in essence unknowable.
A spring is a force member that according to Newton's law changes momentum in a system gradually over time.
Thus,this is why spring force is not considered as an impulsive force or we can also say that, an impulse is a specified applied momentum, and a spring is a specified applied force.