An engineering laboratory tests the dynamic response of various structural components. Which of the following describes a system undergoing a free oscillation?
A quartz crystal vibrating in an electronic oscillator circuit connected to a continuous alternating voltage source.
A marine drilling platform swaying sideways in response to a continuous train of ocean waves of period 12 s12\text{ s}12 s.
A steel cantilever beam that is deflected once by a pneumatic actuator and then left to vibrate.
A loudspeaker cone vibrating back and forth when driven by a 120 Hz120\text{ Hz}120 Hz sinusoidal signal.
