Applets on the 2 kind of SHM are on the forum, "search" the entire forum or just the EJS boards usually gives the applets I am looking for.

simple harmonic motion: simple pendulum
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=818.msg3092#msg3092
http://www.phy.ntnu.edu.tw/ntnujava/index.php?action=dlattach;topic=818.0;attach=470

simple harmonic motion:spring oscillation
http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=349.0

[quote author=kennethkyho link=topic=822.msg3101#msg3101 date=1229350199]
when the mass is too small and the amplitude is large, the mass oscillated like a simple pendulum instead of oscillating vertically.
why ???[/quote]
I  think it is due to the 3-D ( x , y and z) translational degree of freedoms and the rotational degree of freedom cause the vertical pendulum to swing in all possible ways in additional to the up and down you set it to move in SHM. The reason is there may be a slight displacement "off" the line of action of the spring.

[quote author=kennethkyho link=topic=822.msg3101#msg3101 date=1229350199]
also, is there any relationship between the period of the spring oscillating vertically and the period of the "simple pendulum" ???
[/quote]
vertical spring formula is T = 2*Math.PI.Math.sqrt(k/m) valid for F = kx hooke's law only. [img]http://dev.physicslab.org/img/72f3a800-5564-428e-97e2-fb76f4cbef80.gif[/img]
picture taken from http://dev.physicslab.org/img/72f3a800-5564-428e-97e2-fb76f4cbef80.gif
swing pendulum formula is T = 2*Math.PI.Math.sqrt(L/g) valid for small oscillations only. [img]http://www.school-for-champions.com/science/images/pendulum_equations-factors.gif[/img]
picture taken from http://www.school-for-champions.com/science/images/pendulum_equations-factors.gif