Its not only physics, it also has a lot to do with mechanics - that provode perfect application of physics. I know guys as big as John Horsfall that couldn't hit the balls as hard as Francisco Bustamante if they tried. Those same big guys couldn't hit them as hard as Jennifer Chen or Tiffany Nelson either. IMO, and Charlie correct me if I am wrong about this, but I believe Perfect Mechanics + Pinpoint Accuracy = Maximum Transfer Of Energy. We can also look at stroke speed (AKA cue speed) - I have always contended that its not how much power you can muster up, but how much cue speed you can transfer from the cue to the cue ball to an accurately hit rack of balls. IMO, this is where many players get lost with the break shot. That is why I have always said that you should never sacrifice accuaracy for power.
Many players actually zap their strength by trying to generate more power than cue speed. If you watch many players (especially Ewa Laurence) they flinch and tense their bodies prior to maximum follow through of the cue. Ewa's body actually stops at a certain point during her break and you can see her flinching quite clearly. I believe that this "tensing" will eventualy eliminate follow through and prevents maximum energy transfer and accuracy. I'm not picking on Ewa, it is just something that is very evident in her breaking style and her Hall of Fame status proves that this flaw has not held her back at all.
I believe that when it comes down to it, their are players that are "loose" and players that are "tight" in their body mechanics. Rodney Morris has the loosest, smoothest delivery of anyone I have ever seen. Other players that fit into this category are Charlie Bryant, Danny Basavich, Paul Potier, Gabe Owen, Earl Strickland, Mike Sigel, David Howard, Danny Medina, Wade Crane, Ga Young Kim, Helena Thornfeldt, Jeanette Lee, and the late, great Tony Ellin who IMO had the most devastatingly powerful break in the history of the pro tour.
Players that are less fluid, yet they always get a great result would be Johnny Archer, Ralf Souquet, Tommy Kennedy, Thorsten Hohmann, Nick Varner, Fong Pang Chao, Karen Corr, Mike Davis, Allison Fisher, and "Tenessee Tarzan" Mike Massey. Their stroke is abbreviated yet they are always accurate in their delivery which IMO, is more important than blasting power or making a lot of balls.
I learned a long time ago that if you hit the balls too hard, bad things can happen. Clusters can occur, the cue ball can get kicked all over the table or into a pocket, or you can scratch due to an inaccurate contact with the head ball in the rack. In 9 ball, I judge a good break by whether or not the player can control the cue ball and the one ball and pocket a ball all at the same time. I have seen too many players make 4 and 5 balls on the break and not have a shot after the dust settles.
Here are some book excerpts that deal with the subject:
Breaking To Win
Mastering The Break