Last night, we watched a Seven Ball tournament on ESPN2. At one point, a player faced a situation with the cue ball on about the same line as the target ball, thus:
P = Pocket
X = Cue Ball
O = Target Ball
A = Target Aim
P---------------P---------------P
| |
| X-----------------------O|
| A|
| |
| |
| |
| |
P---------------P---------------P
The player played the shot using what the announcer called “strong left english”, hitting the rail at about the target aim spot. Sure enough, the cue ball hit the rail, moved sharply left, and knocked the target ball into the pocket.
The player aimed his cue stick such that the line of the stick was parallel to the line [X,A], but left about an inch, in order to strike the cue ball on its extreme left side.
Okay, here’s my question (finally, eh?). Why did the cue ball go straight along the line [X,A] rather than along the line [X-D,A+D] where D is the distance between the center of the cue ball and its extreme left side?
Put another way, why does a cue ball not veer away from the side where it is struck by the cue stick? That is, why doesn’t the cue ball go along the same vector as if you changed your strike angle such that the spot on the left side were now the center spot? Surely, the cue ball doesn’t “know” from what angle the stick approached when impact was made… or does it?
This is brutally difficult to get into even with your excellent diagram.
I think I get what your saying. You can put spin on the cue ball (which they call english) and have the cue ball travel in a straight line.
I would like to actuially see the shot, but based on your layout, the player would strike the ball to the left side of the ball, creating the spin, and the ball would proceed fairly straight to the bumper, hit it and move left into the target ball. The cue ball would have travelled in a straight line (more or less).
The ball is being propelled straight towards A, with spin created by hitting the left side. It is a spinning top, more or less. It’s gyroscope action will not cause it to veer in any direction, since all foreces are cancelling eachother (all spinning forces). It will proceed straight towards the aim spot.
I have taken numerous shots towards bumpers, with the cue ball spinning on it’s axis yet continuing in a straight line, and then it steers dramatically upon hitting the bumper.
Why? I always saw the ball as a gysoscope, with the spin not creating any steer, and the straightline path as irrelevant to the spin. The spin, however, needs to be delivered right at the equator of the ball to keep the line truest, although I’ve seen players use top/side and bottom/side spins. When they are deilvered, the shooting lines are very difficult to understand and come with years of practice.
I dunno if this will make sense without pics and I can’t find any good links. Hitting the cue on the side does two things: it sets the cue rolling forward and it sets it spinning (around it’s Z axis) clockwise (as viewed from the top in your pic). The Z axis rotation needn’t contribue significantly to the cue balls trajectory until it reaches a bumper.
I can’t tell you why but in order to get a ‘banana’ bend (veer) on the ball you strike (the cue ball) you need to strike it vertically from above. A quarter back and left from the centre of the cue ball generates a swerve to the left before it comes back again.
That doesn’t help too much but it might be a clue.
I’d bet the ball didn’t go quite straight, and hit just left of A, but that most of the action of the side took place on the cushion. You can get some “running side” by hitting this way, but it works over a fair distance (over the length of a billiards table rather than a pool table ;))
This would be for the same reason that when you hit a hard shot with strong screw (ie the bottom third of the cue ball) the ball will hit the object ball, appear to pause, then roll backwards. The effect of spin dominates when the ball has some purchase on the cloth or cushion.
In this case I suspect that the ball was struck sufficiently hard that the effect of the spin – which requires friction to work – was negligible on the journey to A. On the cushion, the spin comes into play equally with the force of the direction of the ball, making the pot.
I think the question he is asking is this which is something I have wondered as well (and if this isn’t his question, then I still want to know the answer )
the 'o’s is the cue ball. If I hit the ball with stick ### at impact point X, the the ball will travel aproximately in the direction of vector R. But If I hit at the same impact point X with stick ===== then why does it go in the direction of vector E. when two balls hit each other, the angle of approach is irrelevant, the object ball will go perpendicular to the tangent at the point of impact. What make a strike by the cue tip different? I have always wondered if it is because the follow-through allows contact (and therefore energy transfer) to occur longer than instantaneously.
The reason is friction between the cue tip and the ball. The cue tip “grabs” the ball, and sets it spinning, while pushing it in the direction of the shot. If there were no friction, the cue could only transmit force normal to the surface of the ball, and the ball would go in the same direction regardless of the strike angle.
I haven’t played for a while, but I don’t recall ball-ball contact causing much english on the impacted ball. That’s because there’s much less friction between two balls than between the cue tip and cue ball.
I think wolfman touched on it. The contact between the ball and the tip of the cue is long enough to propel the ball in the direction of travel, not just directly away from the impact.
I can think of two visualizations that might help. First, think of what would happen if you put something sticky on the tip of the cue. I would expect it to carry the ball slightly before disengaging (and breaking the adhesion might pull the ball off-line). The contact pressure between the ball and cue works just that way (the chalk helps), but seperates without being sticky.
Also, if the ball did move directly away from the point where it was struck, the tip of the cue would ricochet to the other side. That’s a mis-cue. If you’ve ever played you’ve probably done that at least once, and you can feel the cue glance off the ball and to the side. But if you do it just right, you don’t feel the stick being pushed off-line. The trick is learning just how far off-center you can get away with.
For purposes of your question, you shouldn’t look at the cue ball as a ball. Picture it as a child’s top. This is accurate, because in the circumstances of your question, the cue ball does not roll. It slides along the surface of the felt on a single point of contact, just like a top does. It’s attitude is maintained due to the gyroscopic force instilled in the ball as it spins along its horizontal circumference.
The trick to doing this properly is a clean cue strike. The chalk maintains friction so the tip of the cue doesn’t slide along the ball. If it did, it would travel at an angle.
And the ball [d]does** travel at an angle, although a slight one, and not in a direct line from the strike. A good pool player takes this into account when setting up his shot, and aims accordingly.
What is unseen in the shot described is the fact that the billiard player must also take into account the angle of deflection imparted upon the object ball by the spin of the cue ball.
)