Courses: P3, P13, P15
Two upside-down pendula are coupled at the bottom by a rubber band. If you get the right rubber band pressure they will transfer energy back and forth nicely. We generally use them without adding any additional weight to the bobs, but it is possible to add weights.
One can insert a large piece of foam board between the two pendula so that when viewed, the observers see the pendula swinging, then stopping, then starting again... with no obvious cause.
