Forced Oscillations Lab / Pohl's Pendulum

When an oscillating system swings freely, we observe that the decrease in successive maximum amplitudes strongly depends on damping. If the oscillating system is stimulated by an external periodic torque, we observe that in a stable state, the amplitude is a function of the frequency and amplitude of the external periodic torque and damping.

The initial experiments concern free oscillation: Determination of the oscillation period and characteristic frequency of the undamped case. Measurement of oscillation periods and frequencies for different damping values. Calculation of attenuation ratios, damping constants, and logarithmic decrements. Study of the aperiodic case.

Forced oscillations are then studied: resonance curves determined and graphically represented using damping values. Measurement of resonance frequencies. Observation of the phase difference between the torsion pendulum and the external stimulating torque.

Long-lasting oscillation thanks to ball bearings. Damping by abrasion-resistant Foucault current brake. Simple illustration of the basic principle of forced oscillations.