Coupled Series Masses Hanging On Springs
To illustrate coupled oscillations and normal modes.
Two identical rubber corks hanging from two identical small springs as photographed.
Pushing either mass causes oscillations which will couple between the two masses. If the two masses are displaced from equilibrium by the appropriate amount either (1) in phase, or (2) out of phase, the normal modes can be produced. Alternatively, moving your hand up and down at the frequency of a normal mode will excite that mode.
William Pong, Principal Frequencies of a Double Spring-Mass System, AJP 21, 546-548, (1953).