A spring–mass oscillator will have infinite amplitude if excited at its natural frequency, although it takes infinite time for this to build up. When damping is present, the forced response is finite; expressions for both the time taken and the number of forced cycles for this to build up from dormant initial conditions are derived. It is shown that the time taken is independent of the forcing frequency, and hence the final amplitude, while the number of cycles is proportional to the forcing frequency.
