David Alciatore ("Dr. Dave") ILLUSTRATED PRINCIPLES The 90 rule Summary: David Alciatore ("Dr. Dave") ILLUSTRATED PRINCIPLES The 90° rule Part III ­ Carom and Billiards Shots Note: Supporting narrated video (NV) demonstrations, high-speed video (HSV) clips, and technical proofs (TP) can be accessed and viewed online at billiards.colostate.edu. The reference numbers used in the article (e.g., NV 3.4) help you locate the resources on the website. In my previous two articles, I introduced the 90° rule. It states that when the cue ball strikes an object ball with no topspin or bottom spin, the two balls will separate at 90°, regardless of the cut angle. The cue ball leaves along the tangent line, which is perpendicular to the impact line (see Diagram 1). Principle 1 and NV 3.4 summarize and illustrate the important points of the rule. The 90° rule is very useful for helping to prevent scratches (see Part I in the January, 2004 issue) and to plan break-up and avoidance shots (see Part II in the February, 2004 issue). In this article (Part III) we will look at how the 90° rule can also be used to plan carom and billiards shots. We will also see how the 90° rule is important in position play. Principle 1 90° rule With a stun shot where there is no topspin or bottom spin, after impact the cue ball will depart along the tangent line, which is perpendicular (at a right angle) to the impact line. In other words the cue ball and object ball paths, after impact, will be 90° apart (see Diagram 1, NV 3.4 and TP 3.1). 90° impact line Collections: Engineering