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boundary conditions and the Coriolis force
feylike
(no, i have not drunk enough rum to drown a dozen men...)

there is something very satisfying about literally feeling Newtonian dynamics -- particularly angular momentum -- in action in a simple physical system. i've been experimenting with meteor hammers (Chinese martial arts weapon that consists of a rope with a spherical weight at each end) on and off for a while. after having seen video of an acrobatics troupe performing some spectacular tricks with meteor hammers, i tried to replicate some of the simpler motions. one of these was revolving the weights overhead in a horizontal circle with the rope tensioned straight, somewhat like a helicopter rotor.

starting out by grasping the rope with its midpoint in one hand and swinging the weights in a horizontal circle overhead typically means the rope starts twisting up, and the weights orbit directly next to each other. achieving any significant speed requires a large horizontal force to maintain the centripetal acceleration. alternating hands caused the twists to stop forming, but the weights to remain close together, maybe separated by 15 degrees or so in their orbit. eventually i learned to use one hand to speed up or slow down an individual weight (by applying force to the rope near but not at its midpoint) while using the other hand near the midpoint to keep the weights in position. when the weights opened up in their orbit so they were opposite each other in the circle, i felt a most amazing thing: the horizontal force on my hands suddenly dropped to nearly zero, and the ordinarily very flexible rope began to act like a rigid object, needing only a tiny amount of applied force to offset gravity and air drag.

if i had enough space to practice in, i think i could have launched it upward, spinning like a flying disc, and caught it as it came back down. it is probably one of the simplest systems demonstrating angular momentum: two equal masses orbiting circularly around their barycenter, held together by rope tension.

for my main practice set, each end weight is (glass marble in a knot similar to a monkey's fist knot) approx. 30 grams, distance between centers of mass is approx. 72cm. i made a set with 75 gram end weights, but at any usable speed, being accidentally struck with it was very painful.

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