To Bob Jewett

[pooltchr]

Hi Randy,

As others have pointed out, the contact patch is larger according to how much energy is transferred from ball to ball, so a thin hit will have a smaller patch than a full hit, and a harder shot will have a larger contact patch than a softer shot.

One thing to remember in this, that a lot of people seem to be confused by, is that the size of the contact patch has absolutely nothing to do with the aiming accuracy required on a particular shot. The patch width does not give any indication of how precisely the cue ball must hit the object ball to get it into the pocket. In the same way, the contact patch of the tip on the ball gives no indication of how accurate you must be in tip placement to achieve a particular precision of placement of the cue ball.

Bob, I have a follow-up question on this topic. I would assume the contact patch on a straight on hit would be circular in shape. On a cut shot, would it be the same, or would it be smaller on the back side of the collision? (More contact initially, and gradually reducing as the balls separate?)
Steve

 

[randyg]

Thanks Bob.

 

[Bob Jewett]

Bob, I have a follow-up question on this topic. I would assume the contact patch on a straight on hit would be circular in shape. On a cut shot, would it be the same, or would it be smaller on the back side of the collision? (More contact initially, and gradually reducing as the balls separate?)
Steve

I think that the contact patch will be circular for all collisions, but I don't know of any way to see that for non-full collisions other than by a computer simulation. What is a little tricky is that the circle for a cut shot will move some on the ball during contact. That is, the center of the circle will move some on the surface of the ball.

The details of the ball-ball collision are almost insignificant -- in nearly all cases you can treat the collision as instantaneous with an infinitesimal contact point as far as getting the physics right. The exception to this is when more than two balls are in contact simultaneously such as in a rack or for the "ten times fuller" system.

 

[oldroller]

I would

think of it to the extreme,like imagine two basketballs colliding
But i'm not real smart

 

[AuntyDan]

I think that the contact patch will be circular for all collisions, but I don't know of any way to see that for non-full collisions other than by a computer simulation.

I believe I have heard of experiments done with carbon paper taped to the balls which shows the impact point, size and shape. I don't see any reasson it wouldn't work for a cut shot as for a full-on shot, but I've not tried this myself.

Wouldn't the size of the impact deformation affect throw? Throw is a function of the friction between balls at contact, and the larger the contact area the larger the frictional force that can be transferred.

 

[Scott Lee]

Bob...So, if the CB/OB 'contact patch' is larger, based on a full hit or cut angle, how MUCH larger is it? Since the size of the contact patch is "infinitesimal", and the "deformation" is miniscule, would it be measured in fractions of a millimeter, or what?

Scott Lee
www.poolknowledge.com

Hi Randy,

As others have pointed out, the contact patch is larger according to how much energy is transferred from ball to ball, so a thin hit will have a smaller patch than a full hit, and a harder shot will have a larger contact patch than a softer shot.

One thing to remember in this, that a lot of people seem to be confused by, is that the size of the contact patch has absolutely nothing to do with the aiming accuracy required on a particular shot. The patch width does not give any indication of how precisely the cue ball must hit the object ball to get it into the pocket. In the same way, the contact patch of the tip on the ball gives no indication of how accurate you must be in tip placement to achieve a particular precision of placement of the cue ball.

 

[mikepage]

[...]

Wouldn't the size of the impact deformation affect throw?

no.

Throw is a function of the friction between balls at contact, and the larger the contact area the larger the frictional force that can be transferred.

The frictional force is to a pretty good approximation independent of the contact area.

If you slide a 2X4 across a floor, you get about the same resistance whether the "2" side is down or the "4" side is down. When there's half the contact surface, there's twice the weight on each square inch of that contact surface--and these two things cancel out.

 

[dr_dave]

friction and throw don't vary with contact area

Wouldn't the size of the impact deformation affect throw? Throw is a function of the friction between balls at contact, and the larger the contact area the larger the frictional force that can be transferred.

Friction force, and therefore throw, is relatively independent of contact surface area, especially for the small areas and small changes in area that occur with pool balls.

Experimental data agrees with the theory quite well on this. For more info, see my September '06 article.

Regards,
Dave

 

[mikepage]

Bob...So, if the CB/OB 'contact patch' is larger, based on a full hit or cut angle, how MUCH larger is it? Since the size of the contact patch is "infinitesimal", and the "deformation" is miniscule, would it be measured in fractions of a millimeter, or what?

Scott Lee
www.poolknowledge.com

The contact patch is small, but "infinitesimal" is too strong a word I would say. I think the width of the flattened areas can be as big as several mm (several dime thicknesses) on hard shots. This might even affect the cut angle by a few tenths of a degree.

 

[av84fun]

Friction force, and therefore throw, is relatively independent of contact surface area, especially for the small areas and small changes in area that occur with pool balls.

Experimental data agrees with the theory quite well on this. For more info, see my September '06 article.

Regards,
Dave

Wow...that's interesting. I would have thought that a larger contact area would create more friction for a given weight.

But then again, as far as I know, an "engineer" is the guy who drives the train!

(-:

 

[Bob Jewett]

... Wouldn't the size of the impact deformation affect throw? Throw is a function of the friction between balls at contact, and the larger the contact area the larger the frictional force that can be transferred.

That's not the way that friction seems to work. The frictional force seems to be largely independent of the area of contact, or at least that's the way it's taught in high school physics courses. Friction between two objects is specified by "coefficient of friction" and the tangential force is equal to that constant multiplied by the normal (pressing together) force.

More detailed studies of friction find that both speed of slipping and pressure of contact affect the "constant".

 

[Tom In Cincy]

More detailed studies of friction find that both speed of slipping and pressure of contact affect the "constant".

Does that include changes in moisture in the pool room? and if the balls need to be cleaned? or if the contact point just happens to have a little chalk dust on it?

I assume YES but then again....

 

[Bob Jewett]

Does that include changes in moisture in the pool room? and if the balls need to be cleaned? or if the contact point just happens to have a little chalk dust on it?

I assume YES but then again....

It's easy to show that both chalk and saliva make big changes in the ball-ball friction. Humid, dirty conditions seem to increase the friction.

 

[dr_dave]

engineers

as far as I know, an "engineer" is the guy who drives the train!

(-:

That's correct. Gold star! There are also other "engineers" that design the train diesel engines, coupling mechanisms, railroad crossing controls, routing control and monitoring systems, car wheels, bearings, and suspensions, etc, etc, etc.

Regards,
Dave, a pool player and an engineer

 

[AuntyDan]

More detailed studies of friction find that both speed of slipping and pressure of contact affect the "constant".

OK, so then a hard shot would involve more pressure of contact, whic would change the "constant"?

 

[av84fun]

That's correct. Gold star! There are also other "engineers" that design the train diesel engines, coupling mechanisms, railroad crossing controls, routing control and monitoring systems, car wheels, bearings, and suspensions, etc, etc, etc.

Regards,
Dave, a pool player and an engineer

Hope you know I was just kidding and that I respect both kinds of engineers. But personally, I'd rather just own the railroad!!

(-:

 

[av84fun]

Quote:
Originally Posted by Tom In Cincy
Does that include changes in moisture in the pool room? and if the balls need to be cleaned? or if the contact point just happens to have a little chalk dust on it?

I assume YES but then again....

It's easy to show that both chalk and saliva make big changes in the ball-ball friction. Humid, dirty conditions seem to increase the friction.

Right. In Science of Pocket Billiards, Koehler published his study of the effect of various foreign substances on the throw angle.

The worst culprit...by far...was chalk which can cause as much a 9 degree throw angle!!

I think that even pros (with the possible exception of Archer) pay WAY too little attention to chalk marks. Whenever I see a pro miss a fairly easy shot and get that "how the hell did I miss THAT shot" look on their face, I think that a chalk mark is implicated in a LOT of such cases.

Then there are the refs who, when asked to "clean" a ball, do so with their bare, greasy/sweaty hands...which IMHO is an offense that should be punishable by at least a day in jail!!!

(-:

 

[dr_dave]

chalk cling effects

I think that even pros (with the possible exception of Archer) pay WAY too little attention to chalk marks. Whenever I see a pro miss a fairly easy shot and get that "how the hell did I miss THAT shot" look on their face, I think that a chalk mark is implicated in a LOT of such cases.

Cling, caused by chalk smudges, can have a huge effect on both throw and spin transfer. I have some good visual examples in HSV A.142 through HSV A.146. Check them out. The effect is dramatic.

Regards,
Dave