Summary: David Alciatore, PhD ("Dr. Dave") ILLUSTRATED PRINCIPLES
"90° and 30° Rule Follow-up Part II: speed effects"
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.
You might also want to view the resources from a CD-ROM. See the website for more details.
In last month's article, I answered a few basic questions that have come up concerning my
series of articles on the 90° and 30° rules (see my January through June, 2004 articles at
billiards.colostate.edu). In the next few articles, I'll look at the important effects of English,
speed, and other factors on the 90° and 30° rules. If you don't remember what the 90° and 30°
rules are and when they apply, see NV 3.4-3.5 and NV 3.7-3.10. Readers with engineering or
physics backgrounds might also find TP 3.1 and 3.3 interesting. Remember, the 90° rule states
that for a stun shot, where the cue ball is sliding at object ball impact, the cue ball and object ball
paths separate at 90° (i.e., the separating paths are perpendicular). The 30° rule states that
when the cue ball is rolling at object ball impact, and when the cut angle is between a 1/4-ball and
3/4-ball hit fraction, the cue ball's path will be deflected by approximately 30°. If these previous
two sentences are not clear, you might want to look at the online videos and articles mentioned
NV 3.4 90° rule with various cut angles