  We present an instructive way to introduce the subject of resonance using a
typical spring-mass system without damping. In contrast to the standard
approach found in most textbooks, here we propose a way to extend the lectures
from common translational systems to rotating ones. In the latter case,
particular attention is paid to the role of the variable centripetal force. The
students will find analytical solutions of the simplest possible ordinary
differential equations of motion and see that the natural frequency strongly
depends on the rotation speed, the latter being a simple vector sum. The
analysis reveals that always the resultant ground force is harmonic. In order
to depict the significance in the influence of rotation on natural frequency,
we present an imaginary 'antigravity' world in which the aforementioned
dependency is absent.
