Summary: David Alciatore, PhD ("Dr. Dave") ILLUSTRATED PRINCIPLES
"90° and 30° Rule Follow-up Part III: inelasticity and friction"
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 my previous two articles, I answered a few basic questions that have come up concerning
my series of articles on the 90° and 30° rules (see my January '04 through July '04 articles at
billiards.colostate.edu). In last month's article, we looked at the effects of follow, draw, and
shot speed on both rules. In this article, we will look at the effects of ball inelasticity and friction.
In next month's article, I'll conclude the series by looking at the effects of side English.
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
TP 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 above.