Demos: 1D-10 Center of Mass Motion (Air Track)
A pendulum free to oscillate is attached to a rider on the air track. When the pendulum swings back and forth, the rider recoils in such a way that the CM of the floating system remains at rest.
Directions: Test to insure that the track is level by turning on the air supply and watching for any drift of the rider. Once this is done, start the pendulum swinging. Try to avoid giving the pendulum a push so that the CM will remain relatively stationary.
Suggestions for Presentation: Convince the student that there is no NET external force on the system, so acm must be zero. Point out that sometimes we are distracted by the motions of the various parts of a system and have difficulty focusing on something as abstract as the CM. Try placing a mark of some kind at the initial location of the CM and show that the rider oscillates symetrically about this point (if there is no drift).
Applications: (None at present)