3.2 Normal laboratory pendulum and seismographic pendulum in comparison - 1/3

We resume: The seismograph executes a forced oscillation. The way in which this oscillation occurs, however is completely different to what we are used to from 'usual' forced oscillations in a laboratory (see fig. 28).

Fig.28: "Usual" forced oscillations in a laboratory.

The illustration above shows us the familiar scene in which a spring-pendulum is made to oscillate regularly. The upper end of the pendulum is periodically moving up and down. Balancing the forces causing this harmonic movement with we receive for the upper end of the spring