forced osc. and natural f

Q: 我想問下如果我有一個spring system 吊住一個point mass.我拉一下然後放手,佢行唔行natural frequency ?

Yes, the spring-mass system will oscillate in the natural frequency, because the system oscillates on its own. .

Q: 但之後又話under steady state, system oscillate with a frequency equal to external force,又話independent of natural frequency...???!!! 好亂....

I don't have the book, but I think you have misinterpreted the statement.

The statement refers to a forced oscillation system, which is a system that is forced to oscillate by an external periodic force (the driving force). The frequency of the system thus follows that of the driving force.

Q: what is the relationship between forced oscillation , free oscillation , natural frequency , resonance???

Free oscillation is the oscillation of a system under its own restoring force. Under this siutation, the frequency of oscillation is the natural frequency of the system.

Forced oscillation is the oscillation of a system under the action of a periodic driving force. The system oscillated with the same frequency as that of the driving force.

Resonance only applies to a forced oscillation at which the frequency of the driving force equals to the natural frequency of the system. During resonance, the oscillation amplitude reaches a maximum and energy transfer from the driving force to the oscillating system is also the highest.

You could play with the animation given in the following web-page, which helps you to understand forced oscillation:

http://www.walter-fendt.de/ph14e/resonance.htm