# Difficulty - Signals and Systems

• salil87
If energy is added to the system at a frequency that does not match its natural oscillation pattern, it will not be continually absorbed over time. However, if the energy is added at the characteristic mode of the system, it will be continually absorbed and the system's amplitude will increase. This statement may not apply if there is an energy waster in the system, but the overall point is that energy added at the characteristic mode will be absorbed by the system. Fleem provides a helpful example of pushing someone on a swing to illustrate this concept.

#### salil87

Hi
In my notes I came across the following sentences:-
Any signal consisting of the system's characteristic mode is sustained by the system on its own. The system offers no obstacle to this signal.
Not getting what it means. Please, a little help needed. :-)

Thanks
salil

A characteristic mode is simply a natural oscillation pattern the system possesses. If you add energy to a system which is NOT in one of the system's natural oscillation patterns, then that energy will not be continually absorbed over time. For example, if you are pushing someone on a swing and the frequency you are pushing does not match the natural frequency of the swing, then you won't make any notable progress getting the swing going. But if you add energy at the 'characteristic mode' of the swing (the right spectrum of frequencies), then it will be continually absorbed causing the swing amplitude to increase. If there is an energy waster (like an electrical resistor, brake, wind resistance, etc.) in the system then I'm not sure I'd use the word "sustained" as they do in the statement, but I think the general point is that that energy will be absorbed by the system.

I have no idea what that is supposed to mean, out of context. Can you give a longer quote? Is the quote a computer translation from a different language?

Fleem's answer might be correct, or it might not.

Sorry for the short Description. Guess it happens when I'm in tension. :-) But Thanks a lot Fleem ... your explanation helped a lot. Your Swing example rocks. Sometimes I think of such simple things in such complicated ways. Thanks a lot again. :-)
Thanks
Salil