Cue Ball Curve

[Masayoshi]

If you draw your free body diagram, you'll see what's happening.

Let's make sure we're still talking about theoretically being able to shoot with a dead-level cue, which is what the OP was asking, and he qualified the question understanding the difficulties of actually shooting dead-level.

Again, if you shoot upwards, it's easy to see that the cueball will have the OPPOSITE curve (physics agrees with this by torque and the righ-hand rule). Try to make sense of that on your snooker table example. You'll see quite quickly, as I assume you know something about basic physics, that if it curves the opposite direction with an upward stroke vs the downward stroke, then that means somewhere there's an angle of cue that creates no swerve, given the same amount of offset. It stands to reason that this "somewhere" is at dead-level.

Freddie <~~~ no idea what Euler has to do with this

With an upward stroke, it will initially curve in the opposite direction as a normal stroke, but if allowed to roll long enough and assuming the spin didn't wear off, it would curve back in the other direction once natural roll took over.

 

[dr_dave]

With an upward stroke, it will initially curve in the opposite direction as a normal stroke, but if allowed to roll long enough and assuming the spin didn't wear off, it would curve back in the other direction once natural roll took over.

I have never seen this on a level and clean pool table with round and balanced balls. Can you or others making these claims post a link to a video showing this effect or post a new video if necessary?

Regards,
Dave

 

[dr_dave]

Again, if you shoot upwards, it's easy to see that the cueball will have the OPPOSITE curve (physics agrees with this by torque and the righ-hand rule). Try to make sense of that on your snooker table example. You'll see quite quickly, as I assume you know something about basic physics, that if it curves the opposite direction with an upward stroke vs the downward stroke, then that means somewhere there's an angle of cue that creates no swerve, given the same amount of offset. It stands to reason that this "somewhere" is at dead-level.

Well stated Freddie.

Regards,
Dave

 

[dr_dave]

Forgive me if I'm wrong here, but while the force vector you initially exert on the CB in the case of a level cue would not directly induce spin on an axis that is not parallel or perpendicular to the direction of CB travel for as long as the cue is applying force, the CB will, however, as it moves over the cloth, continually shift its axis of rotation because of a friction vector. (In lay terms we say the ball has sidespin and develops then loses natural roll from the cloth, but by Euler's theorem any object rotating around a fixed point like a spinning ball has but one rotation about a single axis -- in our case the resistance of the cloth is a vector along the surface of the ball in the opposite direction of travel and this vector continually shifts the axis of rotation as the CB goes along)

Sure curve is negligible with a level cue and a firm shot (hard for the CB to curve too much when it is partially sliding) and a high degree of sidepsin will tend to stabilize the rotation making the ball travel a little bit like a spinning top/dreidel whathaveyou, but eventually the cloth friction will shift the axis of rotation and the ball will develop roll, and rolling CB with will curve because of sidespin.

Try playing snooker sometime and hit a table-length shot with a level cue with maximum side spin. Especially if you play the shot at pocket speed, you'll miss the ball entirely.

This might happen on a snooker table with a directional napped cloth, but I have never seen it on a pool table. On a level and clean pool table with round and balanced balls, the CB rolls straight with sidespin after any swerve (caused by a non-level-cue hit) wears off (after the CB stops skidding and starts rolling). There are some minuscule physical effects that could theoretically cause a CB rolling with sidespin to turn (e.g., Magnus effects and rolling/spin resistance interaction per TP B-2), but these effects are much too small to be measured or be noticeable on a real pool table.

Here are some pertinent videos for those interested in the topics in the quoted post above:

NV B.10 - Drag spin loss and sidespin persistence, with spin-axis "flip"

NV B.7 - Ball "turn" caused by sidespin

NV B.8 - Straight throw of a second object ball

And much more info can be found on the OB turn resource page.

Enjoy,
Dave

 

[Cornerman]

Oh, you didn't go all Bernoulli on him, did you?

<------has no idea what any of that means. but I'll trust that Cornerman is right

LOL! Nobody should ever just believe.

I was the one that was wrong 20 years ago, until I actually did the analysis.

Freddie <~~~ sometimes has to play engineer