I have been reading "Response of a Single Degree of Freedom System to Undamped system" and have been referring the book on Dynamics of Structures by Anil K. Chopra.I have got the following questions: 1) One , basic question I have is that the author says that an example of a harmonic excitation is the force due to unbalanced rotating machinery. What is the meaning of an unbalanced rotating machinery(am not from a mechanical background!!!) . Can anyone give some more (simple) examples of harmonic excitation??? 2) On solving the governing differential equation of a single degree of freedom system subjected to a harmonic excitation, gives the author mentions that the displacement is said to be in phase with the applied force ( that is in the same direction as the applied force) if the ratio of the forcing frequency to the natural frequency is greater than 1 and the displacement is said to be out of phase with the applied force(that is the displacement is in opposite direction of the applied force) if the ratio of the forcing frequency to the natural frequency is less than 1. Now, my question is , that does this out of phase or in phase have any physical significance whatsoever?