Trace on the screen of the CRO

This is a LRC oscillation circuit. The CRO measures the voltage across the capacitor.

In the absence of a resisotor, the oscillation would produce a sinusoidal wveform. Because the resistor dissiaptes energy in the form of heat, the amplitude of the oscillation decreases gradually with time.

The mechanical analogy of such circuit is a spring-mass system with light damping. The time variation of the displacement of the mass is analogous to the CRO trace.