1. When the body is displaced from equilibrium, there must exist a restoring force... i.e. a force that wants to put the body back in equilibrium.





2. This restoring force must be proportional to the displacement of the body.



hope the answer of the Question

Force should be directly proportional to displacement and should be in opposite direction of displacement.



F= -k x , where F is a force, & x is displacement from mean position, k is proportionality constant.

This needs to be a spring-mass system with zero friction (or damping). Once some external force excites it, it will oscillated forever.

Refer the below link





http://www.citycollegiate.com/shm_XIa.ht...