The Friction Oscillator
The friction oscillator mechanism consists of a rod moving horizontally over two disks separated by a distance ; the disks spin in opposite directions, providing a net kinetic friction with effective coefficient . Kinetic friction (shown in green) is proportional to the weight . The rod moves a distance because the components of normal forces (in brown) are changing.
The normal forces are
and the frictional forces are
 D. Russell. The Friction Oscillator [Video]. (Jul 23, 2013) www.youtube.com/watch?v=46lk2FXzwT8.