How do ball conditions affect the 90° and 30° rules?

Diagram 1 in “90° and 30° Rule Follow-up – Part III: inelasticity and friction effects” (BD, April, 2005) illustrates the effects of inelasticity (i.e., the coefficient of restitution) and ball friction (i.e., throw) on the 90° rule. TP A.5 shows the detailed technical analysis along with example numbers.

Only inelasticity affects the cue ball direction. Friction, from cut-induced or spin-induced throw, theoretically (and practically) has no affect on the cue ball direction for a stun shot (for which the 90° rule applies). Diagram 1 in “90° and 30° Rule Follow-up – Part IV: sidespin effects” (BD, May, 2005) shows the effects of friction and sidespin on other types of shots. The technical details for that are in TP A.7.

For more information on ball elasticity, see the ball elasticity resource page.

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