Summary: David Alciatore ("Dr. Dave") ILLUSTRATED PRINCIPLES
The 30° rule:
Part II Examples
Note: Supporting narrated video (NV) demonstrations, high-speed video (HSV) clips, and
technical proofs (TP) can be accessed and viewed online at www.engr.colostate.edu/pool.
The reference numbers used in the article (e.g., NV 3.8) help you locate the resources on
As I pointed out in last month's article, if you don't know the 30º rule yet, learning it can truly
transform your game. Like the 90º rule presented in my previous series of three articles, the 30º
rule helps you predict the path of the cue ball after impact with an object ball. You may recall that
the 90° rule applies only for stun shots, where the cue ball strikes an object ball with no topspin or
bottom spin. However, with many shots the cue ball is rolling (with topspin) by the time it strikes
the object ball. This is where the 30° rule comes in handy. Last month's article (April, 2004)
introduced the rule, when it applies, and how it is used in practice. The purpose of this article is
to show some real examples of how you can use the rule in your game to prevent scratches, plan
break-up or avoidance shots, and execute carom or billiards shots.
The key points of the 30º rule are summarized in Principle 2 and illustrated in Diagram 1.
The rule states that if the cue ball hits approximately half of the object ball (see Diagram 2), the
cue ball will deflect off at very close to 30° from its original path. An exact half-ball hit, where the
center of cue ball is aimed at the edge of the object ball, is illustrated in Diagram 2.