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

• poolplayer
In summary, the cue ball goes almost straight when its right (or left) side is struck by a cue. This is quite different from when a ball hits another ball, in which case the ball goes almost perpendicular from the contact surface. The ball first slips over the cloth, with or without rotation and independent of where it was hit. Once it hits another ball, friction takes over and the linear movement becomes a rotation, such that the ball rolls from there on.
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.

DWT said:
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,

DWT said:
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.

poolplayer said:
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://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!

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.

dejarnett said:
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.

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