A ball struck by a cue in billiards with English goes straight at first....

[A.T.]

Until then you can search for using a phenolic tip. These are the lowest friction tips on the market

Note that "lowest friction tips on the market" doesn't mean "no friction". If friction was irrelevant to ball direction, it should work even when the low friction is tip lubricated.

 

[DWT]

Thank you for your answer. My thought is that as long as we don't miscue, there is enough friction between the tip and ball due to chalk.

But, I have never tried English with a phenolic tip without chalk. If we can make the ball go straight without miscue, it might not be friction because now the friction should be same as ball-to-ball collision. To avoid miscue I guess we can only strike around the center without chalk though.

We will not be able to put anything more than a tip of english for sure bu

 

[DWT]

We will not be able to put anything more than a tip of english for sure bu

Above are links to videos showing results of the experiment.
Made a tip out of a cue ball, was not able to put anymore than a tip of english on the ball or i would miscue.
Was not able to get spin on the ball due to lack of friction but the angle of travel was really consistent.

 

[poolplayer]

Made a tip out of a cue ball, was not able to put anymore than a tip of english on the ball or i would miscue.
Was not able to get spin on the ball due to lack of friction but the angle of travel was really consistent.

Did you make the tip from a cue ball?? So cool!
It's hard to tell, but it looks like the ball goes to the normal direction rather than parallel to the cue direction... What do you think?

I have a break cue with a phenolic tip, so I could also try some experiments when I have a chance. Though I am not sure if the phenolic tip is the same material as the cue ball (probably different, right?)...

 

[DWT]

Did you make the tip from a cue ball?? So cool!
It's hard to tell, but it looks like the ball goes to the normal direction rather than parallel to the cue direction... What do you think?

I have a break cue with a phenolic tip, so I could also try some experiments when I have a chance. Though I am not sure if the phenolic tip is the same material as the cue ball (probably different, right?)...

Yep here's the cue ball i used.

The ball is splitting the angle from the natural to parrallel to the cue very consistently.

Phenolic tip i think is a little softer than the stone balls but i do not have anything to test that.

 

[DWT]

I have a better way to test this which will remove any potential human error.

I am going to take a ball and core it out to reduce its mass, we will see what happens when the object ball is struck by a ball identical in every way except its weight. This may take me a day or so i will post video of results asap.

 

[poolplayer]

I have a better way to test this which will remove any potential human error.

I am going to take a ball and core it out to reduce its mass, we will see what happens when the object ball is struck by a ball identical in every way except its weight. This may take me a day or so i will post video of results asap.

Wow. I don't know how you can core it out, but it's going to be great to see if you are right and the ball goes straight when hit by the light ball!

Just some quick comments:
In your last video, the impact point was a bit unclear, but it seems that the ball did not go parallel to the cue direction. So, I believe that friction is indeed the key factor. Also, your cue stroke seemed to go off a bit to the side after impact (I believe it is to avoid miscue) and that may also affect the ball direction. I was wondering if we can stop the cue stroke at the contact point, which might make our interpretation easier. Maybe I can try that when I have time.

 

[DWT]

I think we may find out that it is a combination of friction and mass. After I use the cored out ball i can put some chalked felt or maybe leather at impact point. That should give same results as hitting with chalked cue tip.

 

[poolplayer]

I think we may find out that it is a combination of friction and mass. After I use the cored out ball i can put some chalked felt or maybe leather at impact point. That should give same results as hitting with chalked cue tip.

That's a great idea!

By the way, I just found this .pdf where a guy from Argonne National Laboratory explained the cue ball direction with friction as I did (please see p.19).
ftp://ftp.tcg.anl.gov/pub/shepard/pool/old_versions/physics.PDF
But, a model is just a model. Nobody has done your experiment. It's exciting to see what happens.

 

[poolplayer]

I think we may find out that it is a combination of friction and mass. After I use the cored out ball i can put some chalked felt or maybe leather at impact point. That should give same results as hitting with chalked cue tip.

Maybe you can apply chalk between the balls if felt or leather does not work well.

 

[DWT]

New thought just occurred to me the circumference of the cue tips are generally the same as either a dime or nickle. When i used the cue ball tip it has the same circumference as the balls.
If I lined the center of the cue ball to the contact point the ball would not hit the contact point it would hit inside the contact point closer to the center of the ball sending it on a similar path as to what we observed in the above videos.

I am going to redo the tip test using a glass marble it will be more accurate to the actual shape of a normal tip.

poolplayer thanks for the link after that I am fairly sure that the mass of the ball should not effect the angle but I will do the experiment anyway.

Got to love this game its a 4-dimensional game played with 3-dimensional objects on a 2-dimensional surface and in its simplest form your just trying to pick the right 1-dimensional line to hit the ball.

 

[A.T.]

I am going to redo the tip test using a glass marble

I would also suggest some lubricant to make friction as low as possible,

it will be more accurate to the actual shape of a normal tip.

The tip size might also affect friction, even for the same material coupling. The assumption of a constant friction coefficient, regardless of pressure, is just an approximation.

 

[Mark Davidson]

Hi, let me ask probably dumb questions to physics experts... Why does the cue ball goes almost straight when its right (or left) side is struck by a cue (right English)? This is quite different from when a ball hits another ball, in which case the ball goes almost perpendicular from the contact surface. Here, I want to focus on the ball direction immediately after the impact and ignore the curve after the ball starts to roll.

I heard that it is because the ball and cue can be thought as unity so the force only propagates to the cue direction. But if so, would it be possible that the cue makes the ball spin? My guess is that the friction between the ball and cue tip makes the ball not only spin but also go to the right direction and counteracts the force perpendicular from the contact surface (left), resulting that the ball goes to the cue direction? I heard that the friction coefficient is pretty high ~0.6. Any comments will be appreciated!

The 6 corners (or vertices) of a regular octohedron are equally spaced on a sphere. The measle ball dots must be the vertices of a regular octohedron. Here are some links on the geometry:
https://study.com/academy/lesson/octahedron-definition-properties.html
https://en.wikipedia.org/wiki/Octahedron
http://www.math.ubc.ca/~cass/courses/m308/projects/cchang/webpages/octahedron.html
http://mathworld.wolfram.com/RegularOctahedron.html

Good luck!

 

[dejarnett]

Thanks! So from another thread - basically make a box that the cue ball will sit inside and drill a hole in the middle of each side.

 

[Mark Davidson]

Thanks! So from another thread - basically make a box that the cue ball will sit inside and drill a hole in the middle of each side.

That sounds like a really easy way to do it.