To illustrate a system with two pendular modes of oscillation.◙
Bifilar pendulum as photographed.
Both lengths are adjustable.◙
A ball is suspended by a string attached to another string which hangs from a support rod as shown in the photograph. In one direction the pendulum swings with a short length while in the orthogonal direction it swings with a much longer length. When the ball is moved in any other direction and released the resulting motion is a combination of these two orthogonal modes.
Paul F. Bartunek, Some Interesting Cases of Vibrating Systems, AJP 24, 369-373, (1956). ◙John W. Then, Bifilar Pendulum - An Experimental Study for the Advanced Laboratory, AJP 33, 545-547, (1965). ◙Richard M. Sutton, An Experimental Encounter with Bifilar Pendula, AJP 21, 408, (1953). ◙S. M. Lee, The Double-Simple Pendulum Problem, AJP 38, 536 (1970).