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Summary: David Alciatore ("Dr. Dave") ILLUSTRATED PRINCIPLES
The 90° rulePart I the basics
Note: 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 correspond to the numbers used on
the website.
A very important skill for
aiming and planning shots is
the ability to predict where the
cue ball will go after impact
with an object ball. Diagram
1 and Principle 1 summarize
one of the most important
principles of pool related to
this the 90° rule. It states
that when the cue ball strikes
an object ball with no topspin
or bottom spin, the two balls
will always separate at 90°. In
other words, the tangent line
will be perpendicular to the
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