Unsapien,
Frame rate was found by timing the whole video, and then running it frame-at-a-time and counting frames, and dividing.
The sling rotation rate at release, is the only thing I care about; wind-up rotations are typically much slower. What I did was, pause the video just before the slinger released, and then step it a frame at a time until the release, then back up one frame and compare the sling's angle in that frame, to the angle in the release frame.
As an example, Mark Weaver published several videos of himself demonstrating the figure-8 style with tennis balls, in
his article
. I determined that the video was recorded at about 15 frames per second. When he demonstrated a power throw, the frames of interest bracketed the release; just before the release, the sling is just rising from behind his back; in the next frame, the sling is above and in front of him, having rotated through about 180 degrees. 180 degrees in a 15
th of a second, works out to 7.5 rotations per second.
In
other videos
, David Taylor spun his sling through 90 degrees in a 30
th of a second, which is also 7.5 rotations per second.
Hmmm... interesting. Both Weaver and Taylor were doing 7.5 rotations per second. I though it was 10. I assumed I could do 10 revs/second because that's what I thought Weaver and Taylor did. But they didn't ... so I probably don't. So my calculations are wrong, the stone's centripetal force will be less. About half of what I originally calculated, in fact. Instead of the stone pulling with a force of 682 newtons (153 pounds), it will pull with a force of 384 newtons (87 pounds).
And here I've already bought a set of those 200-pound snap swivels....
At any rate, this is enough about snap swivels. This thread was about TechStuf's cool sling grip, until I hi-jacked it. We now return you to the original topic.