I am having trouble understanding resonance. But before that I need to understand natural frequency of an object. I saw similar questions posted in the forum, but I dont understand the answers fully. So what exactly is a natural frequency? One answer which I have read says "Nearly all objects, when hit or struck or plucked or strummed or somehow disturbed, will vibrate. If you drop a meter stick or pencil on the floor, it will begin to vibrate. If you pluck a guitar string, it will begin to vibrate. If you blow over the top of a pop bottle, the air inside will vibrate. When each of these objects vibrate, they tend to vibrate at a particular frequency or a set of frequencies. The frequency or frequencies at which an object tends to vibrate with when hit, struck, plucked, strummed or somehow disturbed is known as the natural frequency of the object." So is the natural frequency always present in an object? Like for instance a tuning fork, does it have a natural frequency when 'not' struck? Also if i hit it, i will have a frequency, but then if i hit it harder, there will be more ossicliations per unit cm, so the frequency will get higher, wont it? So the natural frequency, if there is one, changes?