Resonance -- How does it occur? I am having a little difficulty with the concept of resonance. Could someone explain how resonance occurs and how elastic oscillation relates to resonance?
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Qualitatively, it is easiest to consider a simple system. Suppose you have a mass on a spring. If you stretch the spring and let go, the mass oscillates, that is vibrates up and down. The mass will vibrate at a fixed frequency. This is the often called the natural frequency. Now suppose we force the mass using a motor (or our hand to vibrate at the resonant frequency (approximately the natural frequency, the details are complicated)). The amplitude (i.e. degree of stretching of the spring) attains it's largest value. Forcing at higher or lower frequencies may not achieve as great a stretching. There is more but you may want to read up or reframe your question for more detail.
Every resonant process is a process which can store energy. If a process cannot store energy, then there cannot be a resonance. There are always limits to how much energy can be stored by a resonant process. When these are exceeded you get a breakdown. For example, the Tacoma Narrows bridge disaster was the result of a resonant process: http://en.wikipedia.org/wiki/Tacoma_Narrows_Bridge_(1940)
Hold a slinky by the top and let it dangle. Give it a jerk then hold your hand still and you should see the other end oscillate. That's the natural (resonant) frequency for the slinky. If you shake the slinky top up and down at that frequency, the other end oscillates with a large amplitude. If you go too fast or too slow, the slinky doesn't absorb energy, and the bottom of it doesn't move up or down nearly as much.