Clamp the chin rest (a cross bar on a bench rod) to a moveable demo cart. Adjust the height so that the professor's chin comfortably touches it and so that the professor's nose just touches the bob. Wear the baseball mask, release the bob and don't move. The bob loses about 1/2 inch per swing. Warning: The bob has a mass of 7kg. Don't impart any additional momentum to the bob when releasing it. Don't lean forward while it is moving.