Q: A (pointlike) fly is sitting at a corner of a 2 x 1 x 1
brick. What's the furthest point from it, measuring along the
[rec.puzzles, 15 Apr 1996]
A: Suppose it's at the origin, with a long edge of the brick along the positive x-axis and other edges along the positive axes. Then the furthest point is (2, 3/4, 3/4) which is at distance sqrt(65/8) = 2.850+.
Some details: consider the x-y plane to be
horizontal, and suppose we're observing the brick from y =
-infinity, so that for example the Right face is x = 2, and
the Top face is z = 1. Then the shortest route to points on
the Top face is
Front-Top so sqrt(x2 + (1+y2)) if y <= x
Left-Top so sqrt((1+x)2 + y2) if y >= x.
By symmetry the shortest route to points on the Back face is
Bottom-Back so sqrt(x2 + (1+z2)) if z <= x
Left-Back so sqrt((1+x)2 + z2) if z >= x
and the shortest route to points on the Right face is
Front-Right so sqrt((2+y)2 + z2) if y <= 3-3z and y <= z
Front-Top-Right so sqrt((1+y)2 + (3-z)2) if y >= 3-3z and y <= z
Bottom-Right so sqrt(y2 + (2+z)2) if z <= 3-3y and y >= z
Bottom-Back-Right so sqrt((3-y)2 + (1+z)2) if z >= 3-3y and y >= z.
Although intuitively one expects the furthest point to be the far
corner (2, 1, 1), this is at distance only sqrt(8) = 2.828+. The set of
points further away than this form a kite-shaped region on the Right
face. The kite's long axis lies along the line y =
z, with the head at (2, 0.73205, 0.73205) and the tail at (2,
1, 1), and the cross-piece ends at (2, 0.72486, 0.75838) and (2,
0.75838, 0.72486). The kite is bounded by the concave circular arcs
(1 + y)2 + (3 - z)2 = 8
(3 - y)2 + (1 + z)2 = 8
(2 + y)2 + z2 = 8
y2 + (2 + z)2 = 8.
Q: ??? On a centrally-symmetric body, must it be the case that every, or indeed any, longest geodesic's endpoints are antipodes?
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