SHM Chapter 07 define SHM

Definitions, equations and graphs 

            Definition of Simple Harmonic Motion (SHM):           LO (d)

http://dl.dropboxusercontent.com/u/44365627/lookangEJSworkspace/export/ejss_model_SHM07/SHM07_Simulation.xhtml

A motion where the  acceleration a of an object is always directed towards a fixed point ( the equilibrium position O ) and proportional to its displacement x from the fixed point is said to be  be simple harmonic.



        Defining equation of SHM
a =  - ω² x
Observe from the model or the animation, suggest the reason for the negative sign.
The negative sign means a is always directed towards the equilibrium point while x is pointing away from equilibrium point O.

In other words a and x are in the opposite direction.                                         

Model:

http://dl.dropboxusercontent.com/u/44365627/lookangEJSworkspace/export/ejss_model_SHM07/SHM07_Simulation.xhtml