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Hoop Dreams (Posted on 2006-11-10) Difficulty: 4 of 5
I am dreaming of three balls on a hoop.

Three solid balls of radii a, b, and c are placed on a hoop of radius d. The hoop is a thin, rigid wire. The balls have been drilled so they fit perfectly on the wire hoop and slide without friction. The hoop passes through the center of gravity of each ball. The hoop is oriented vertically so the balls slide to the bottom.
The following information is known:

1) a, b, and c are all < d,

2) d is large enough so that each ball is entirely below the center of the hoop,

3) the central ball is large enough so that it touches the other two balls,

4) the balls are made of the same material so that their weights are proportional to their volumes,

5) the forces that the balls exert on each other and the hoop are directed along the lines determined by their centers.

After the balls come to rest, what is the angle between the vertical and a line from the center of the hoop to the center of each ball?

Inspired by Three Balls in a Bowl.

No Solution Yet Submitted by Larry    
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Solution First Cut at Solution | Comment 1 of 2
Here's my first cut at a solution.
Let A, B, and C be the centers of the balls
with radii a, b, and c respectively. Their
weights are a constant k times their radii
cubed. Let H be the center of the hoop and
L the lowest point on the hoop. Define
    t_X = <LHX  for X = A, B, and C
Assume ball with center B lies between the
other two balls. Thus
    t_A = t_B - d_A
    t_C = t_B + d_C
    (a+b)^2 = 2d^2[1 - cos(d_A)]
    (b+c)^2 = 2d^2[1 - cos(d_C)]
We want to maximize the following,
    k*a^3*d*cos(t_A) + k*b^3*d*cos(t_B) + k*c^3*d*cos(t_C)
    k*d*[a^3*cos(t_B-d_A) + b^3*cos(t_b) + c^3*cos(t_B+d_C)]
This occurs when,
    a^3*sin(t_B-d_A) + b^3*sin(t_B) + c^3*sin(t_B+d_C) = 0

                   a^3*sin(d_A) - c^3*sin(d_C)
    tan(t_B) = -----------------------------------
                a^3*cos(d_A) + b^3 + c^3*cos(d_C)

  Posted by Bractals on 2006-11-10 15:27:52
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