Shawn:
Excellent post, but be careful with this argument, for it can be turned around on you. The kinetic energy formula certainly holds true. Yes, if you can accelerate any material fast enough -- regardless of hardness/softness of that material -- you can have that material shatter, penetrate, or damage other harder materials. One can argue that if you can accelerate a normal playing cue with a soft leather tip (e.g. ElkMaster) fast enough, it too can damage a cue ball. I recall there was an episode of Mythbusters (one of my favorite TV shows) whereby the "myth" of a drinking straw being thrust into or through the trunk of a small tree during a tornado was explored. Long summary short, Mythbusters proved this myth "plausible" because they were able to recreate it, but it took an ungodly amount of power to do so -- almost out of the range of human technology's ability to deliver. Yet, by just increasing the hardness of the projectile (i.e. they used wooden coffee stirrers, and then graduated to lengths of piano wire), the amount of energy needed to thrust that projectile into the trunk of the tree diminished dramatically.
Hardness/softness of the projectile material (i.e. break cue tip) does play a factor, in that the threshold for damage against the harder material (cue ball) is questioned as to whether that threshold is within a human being's ability to deliver that amount of kinetic energy. It is well-known that a plain leather tip is soft enough that it raises the threshold [of damage to the cue ball] out of the range of a human being's ability to deliver. At least any pool-playing human of recent human memory.
Hope this is helpful,
-Sean
