From http://everything2.com/title/Newton%2527s%2520Cradle: The hand that rocks the (world's biggest) cradle
Wiki also tells us of the world's biggest set of balls. Up to twenty full-sized bowling balls lined up in the classic cradle arrangement, suspended from girders by aircraft grade steel cables. To be honest, they usually only run it with 16, as the building shakes a bit too much with all 20 in operation. This also leaves four spares in case of a disaster, but it still looks impressive, even on a web page.
"The problem," according to builder, Mark Broker, "was having the balls to build it."
After a number of trials, the team started working with AMF, a leading supplier of bowling balls, and AMF made them a set of 20 balls matched for weight (15lb each) and size and internal composition.
I can't find a video on the web, but the website (the Geek Group) is listed below, and the installation is at 2309 N Burdick St, Kalamazoo, Michigan.
The guys at Geek Group say,
"In the 2008 season we are working to secure a larger facility for the cradle and rebuild it to an even larger setup with higher-weighted balls and many more of them. We hope to not only set a world's record with this, but to break our own existing record."