My textbook says, "Very often the body undergoing periodic motion has an equilibrium position somewhere inside its path." " If a body is given a small displacement from the equilibrium position, a force comes into play which tries to bring the body back to the equilibrium point giving rise to oscillations and vibrations." I have the following doubts- a) What do they mean by "very often"? I can only think of circular motion as the only case where the equilibrium position is not inside its path? Am I correct? And what are some other examples of periodic motion which are not oscillatory motion? b) When we are giving the body a small displacement, aren't we applying a force, and wouldn't this force be responsible for giving rise to oscillations and vibrations. Why did they say that the force comes into play after the particle has been displaced? In the example of a motion of bob of a pendulum, what forces cause the bob to move for so long? I am new to this topic, so please explain as simply as possible.