Apologies in advance if this is too basic a question for Phys.SE. I don't want to dumb down this venerable institution. :)
My wife and I just watched this TV ad for Kit-Kat where a crew of crane operators line their wrecking balls up to form an enormous Newton's Cradle. It left us wondering if the basic premise is feasible. If the wrecking balls were perfectly aligned would it work? I imagine the lack of lateral stability inherent in having only one anchor point per "ball" would be a problem, but what else (if anything) stops this from working for real?