Why do people overcut cut shots (i.e., hit OB ball too thin)?

Sometimes the aim is just off. When using english (especially outside english), swerve and/or throw might be the culprit. For more info, see:

aim compensation for squirt, swerve, and throw

On firm follow shots and/or jump shots, sometimes the OB is overcut because the CB is airborne when it hits the OB. For more info, see:

from Bob Jewett:

There is some throw on all shots. That is, the surface of the cue ball is somewhat sticky, and as it passes across the surface of the object ball, it will pull the object ball off the ideal path, which is the line through the centers of the two balls at the instant of contact.

If the cue ball is moving faster, there is not as much friction. This is contrary to the simple explanation of friction which says that the coefficient of friction does not change with the force of the contact. The reduction seems to be over a factor of two as you crank the speed up

If you have stun on the cue ball (no follow or draw), the throw you see on a cut shot is maximized for that speed because all the rubbing is to the side. As you add draw or follow, the effective friction is reduced because part of the rubbing is up or down, and that doesn’t contribute to the throw.

Most players have no idea that this stuff is going on, and to some extent it is better for them not to know. They learn subconsciously to make corrections for it, and if they start thinking about it during shots, their brain will get in the way of their arm.

There are fairly simple demonstrations for this. It is discussed on Dr. Dave’s throw tutorial page and in Bob Jewett’s May ’06 BD article.

The cue ball also jumps on hard shots as mentioned above, but this causes thinner cuts mostly for short distances to the object ball or excess stick elevation.

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