in my knowledge,
there are two kinds of simple harmonic motion...

one is simple pendulum, the other is spring oscillation...

and one in school,
i was doing experiment of spring oscillation...

a light spring was hang on a retort stand and a slotted mas was suspended with hanger from it.
and the mass started oscillating vertically with a small displacement.
and i need to court the period of the 50 oscillations.
then i just repeat the timing with different mass.
and finally i need to find the force constant and effective mass of the spring.

[color=red]here's the question...[/color]
when the mass is too small and the amplitude is large, the mass oscillated like a simple pendulum instead of oscillating vertically.
why ???
also, is there any relationship between the period of the spring oscillating vertically and the period of the "simple pendulum" ???