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Snookered! (Part I) (Posted on 2007-01-01) Difficulty: 3 of 5
Billiards experts work their magic on a pool table without any exact knowledge of distances, angles or speeds. Can you?

A regulation billiards table is twice as long as it is wide, has six pockets in the conventional positions, and a total of 18 “rail sights” (the little aiming circles or diamonds along all four sides). Rail sights and pockets form 24 equally spaced divisions around the table. You are at the “head” of the table, and desire to hit the cue ball from against your rail (the head rail), over the “foot spot” (that little dot near the other end), off the far (foot) rail, and back into one of the corner pockets on your end. For non-experts, (including me!), the foot spot is always two rail sight marks from the from the foot rail and centered between the two long sides.

Assume the following:
The ball rolls only – no sliding, no “english”, no leaving the table, therefore all shots go straight since the table is level Coefficient of friction of the rolling ball on the table is 0.05 (a realistic value)

The ball always makes a perfect collision with the bumper (i.e. no energy loss and no friction while in contact) Ignore the size of the ball and pockets – treat both as point entities.

Mass of the ball = OOOPS!, some pool shark has switched your cue ball for one of unknown weight (mass).

Q1: Where do you place the cue ball in order to make the bank shot?

Q2: What is the minimum initial speed you must give to the ball on this trajectory in order to make the shot (assume that if the ball stops exactly at the pocket, you succeeded)

  Submitted by Kenny M    
Rating: 3.0000 (1 votes)
Solution: (Hide)
Note: Scaling from table widths to rail sight distances leads to Charlie's solution and the one below being identical.

A1:

Let s = distance between rail sights (= distance between rail sights and adjacent pockets). Then the table is 4s wide and 8s long, and the foot spot is 2s from the far end and 2s from either side.

A perfect collision implies that the angle of incidence = angle of reflection at the rebound point on the foot rail.

Let x = a direction along the width of the table.
Define x1 = the x distance from the foot spot to the rebound spot on the foot rail.
Define x2 = the x distance from the rebound spot on the foot rail to the side rail

Then x1 + x2 = 2s (1)

Using similar triangles:

x1/2s = x2/8s (2)

Solving (1) and (2) together gives x1 = 0.4s, and x2 = 1.6s Moving back to the head rail (your end), the cue ball should then be placed 2 * 1.6s = 3.2s = 3.2 rail sights from the target pocket.

A2:

Assuming an initial speed, (v), the only retarding force is friction (f).

One method to use is to note that the initial speed imparts kinetic energy (KE) to the cue ball, and the friction force (f) does work to retard the speed as it acts along the length of the path the ball takes from rail to rail to pocket (call this path length z). If the initial Kinetic Energy = Work done by friction, then the cue ball speed will be exactly zero at the target pocket.

KE = (work done by friction, WF) = WF (3)

KE = 0.5 * (mass of ball, m) * v^2 = WF = f * (path length, z) = f * z (4)

Also

f = (coefficient of friction, mu) * (normal force of ball on table, N) = mu * N (5)
&
N = constant = weight of ball = (mass of ball, m) * (gravity constant, g) = m * g (6)

Combining

0.5 * m * v^2 = mu * m * g * z or v^2 = 2 * mu * g * z (note mass cancels out!) (7)

Then, remembering from part one, that the path of the cue ball is an isosceles triangle with base 3.2s and altitude 8s;

z = 2 * ( (8s)^2 + (1.6s)^2 ) ^.5 = 2 * (66.56)^.5 * s

Therefore v ~ 4.0002 (s)^.5 m/s (s in meters), or ~7.2485 (s)^.5 ft/sec (s in feet) Authors Note: Now think about adding in “english”, cue balls that can also slide, multiple bank shots, etc. Pretty amazing!

Comments: ( You must be logged in to post comments.)
  Subject Author Date
Some ThoughtsPuzzle ThoughtsK Sengupta2023-02-11 22:23:05
Solutionre: solutionCharlie2007-01-06 11:16:34
Hints/Tipsre: solutionKenny M2007-01-05 21:40:08
SolutionsolutionCharlie2007-01-01 12:36:58
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