Ball Deflection Made Easy to See
1) Still question at what point the cue ball deflects off the tip with side spin. Set the cue ball against the rail as shown at left on table (1). Make sure the shaft is absolutely parallel with the rail. Shoot center ball and send the cue ball all the way down the rail into the top corner pocket. Easy? You bet.
Now set the same shot up again. Add right spin, but still keeping the shaft absolutely parallel with the rail. The cue ball will be deflected into the rail by the tip causing it to rebound off the rail. The more side spin, the more deflection into the rail. Many straight down the rail shots are missed because of this deflection, so when the cue is frozen to the rail, make sure you execute a clean center ball hit.
If both the object ball and cue ball are frozen to the same rail, with the object ball 4 or 5 feet away, try using a little inside english with a very slightly elevated butt (about 2 inches.) Shot properly, even if the cue ball leaves the rail slightly (1/8"), it will curve back in time to hit the object ball just right. A good way to keep the cue ball from nipping the corner of the center pocket on the way up the rail, it is especially useful if you know in advance that the rails on either side of the center pocket are not proper lined up.
A Fun Shot Using the Set Up At Left
2) Set up the ball as on table (2.) Put an object ball in the jaws of the cross table center pocket. Using hard bottom right, but keeping the shaft parallel with the rail, stroke as if to sink the ball in the upper corner pocket as shown on table (1.) With enough spin and at the right speed the cue ball will spin dramatically off the short rail and take off at a 45 degree angle (+-) to sink the object ball.
This is not very practical in a game, but it will help you understand the physics involved. If you pay careful attention, even to dynamics that you may not use often, there will be times a slight adjustment to a "trick" you've practiced can get you out of trouble.
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