Re: [math-fun] rolling a ball along a fractal
This seems easy - since the limit of the length is infinite, you could only get a fixed point as you go from level n to n+1 if there's some very nice (rational) relationship between the size of the ball and the length of the level n snowflake.
Here's another way to state Josh's point: "How can you roll a ball along a Koch curve when you can't even roll a *hoop* along a Koch curve?" (If we mark a point on the hoop, and roll it along a polygonal approximation to the curve starting with the marked point touching the plane, then when the hoop has finished its journey, the angle between the marked point, the center of the hoop, and the new point of contact between the hoop and the plane will be L/R mod 2 pi, where L is the length of the polygonal approximation and R is the radius of the hoop. But there's no reason to expect L/R modulo 2 pi to have any kind of good limiting behavior as we consider a sequence of polygonal approximations with L going to infinity. Indeed, by continuity, we can construct polygonal approximations that will make L/R mod 2 pi equal to any angle you like.) Jim Propp
Here's an easier one: can you roll a hoop with r>=1/6 (so that it can cross the largest gap) over the Cantor set? With that set, you necessarily have only single points of contact for the hoop; I assume that the edge of the hoop "sticks" to the point and the diameter traces out a circular arc until the hoop hits the next point. No matter what radius you choose, near the beginning of the trip, the edge of the hoop is very nearly flat compared to the distance d between points, and so the hoop will rotate by an amount very close to 2pi*r/d radians. The missing rotation as compared to rolling along a line (which we can define as the limit as d->0 of "sticky" rolling over a lattice with spacing d) clearly converges. If r is big enough then I claim that rolling the hoop over the Koch curve is identical to rolling it over the Cantor set (once if you go across the bottom, twice if you go across the top), since the hoop can't reach the concave parts of the fractal. -- Mike Stay - metaweta@gmail.com http://math.ucr.edu/~mike http://reperiendi.wordpress.com
participants (2)
-
James Propp -
Mike Stay