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When a block of mass m is suspended by vertical spring system,it for sure perform SHM in the absence of damping force,only force which act on the system is internal which is -kx, where k is spring constant,so well so good but there is external force on the system which is gravitational force, which is mg (where g is acceleration due to gravity, for good approximation near earth's surface) which always there on the system towards vertically downward direction.

so what is this force {gravitational force} we should consider of, is it applied force on system to have forced oscillation or damping force ?

If it is applied force then it must be periodic and varies as time passes,but it isn't. A follow up question is that is applied force can be constant throughout to have forced oscillation ? ,if that so,then sorry for the above part !! Then another question arises, On considering this fact as mg act always vertically downwards,Then oscillation with constant amplitude therefore,not sustained,and this situation only arises when damping force is there,but we aren't consider this force to be damping one ?

If it is damped force then it must oppose the velocity for all the time,but it opposes the velocity for sometime and increase the velocity in some part of time.

so if we consider gravitational force then what this force actually doing is in SHM ???

sorry for the lack of -kx sign and upward motion in the diagram Take a look

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Gravity pulls stretching the spring down to a new equilibrium position. This equilibrium stretch gives an upward force which balances and cancels the gravity force. The mass when disturbed from the new equilibrium always pushes back toward equilibrium. The Hooke proportionality is still the spring constant k assuming the spring didn’t get stretched too much and go beyond its proportional limit.

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With a mass hanging on a spring, the only effect of gravity is to determine the rest position. Damping is caused mainly by friction with the air which is always opposite to the direction of motion, and possibly to heat generated in the spring as it undergoes flexing.

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