Two Charged Conducting Pendulums
This Demonstration shows the motion of two pendulums coupled by their electrostatic charges. These pendulums are assumed to be perfect conducting spheres suspended by massless rigid rods. The equations of motion go beyond the linear approximation. The forces considered are gravitational force, electric force, resistive force, and tension. The induced polarization is considered using the method of images up to the first order.
 R. P. Feynman, R. B. Leighton, and M. Sands, The Feynman Lectures on Physics, Vol. 3, 2nd ed., Reading, MA: Addison Wesley, 2005.