Here's a thought I had for measuring the often cussed and discussed subject of stone spin on release.

Almond-shaped glandes buzz. That buzzing is believed to come from rotation as the two 'wings' of the glandes rotate. Much like one half of an old-time mechanical siren.

If one were to measure the pitch of the buzzing, allowing for doppler effects, and then divide by two (there are two wing pulses per revolution) I think you'd have the rotational speed.

Perhaps a spherical stone can be made to do the same somehow?

comments?

I made up some 80gram plaster of paris "eggs", painted one side white and one side red.   In the spring/summer, I was going to have my son do a movie of these on launch.   If we can take the movie fast enough, then play it slow enough, we should have a picture of the rotation/tumbling of one glande on launch.     mgreenfield

And, knowing the movie's frames per second, you might be able to count the stone's rotation. If your camera has a 'slo mo' option use it, it makes the camera run faster and counting easier.

a quick thought here
to measure sling speed videoagainst a  back drop  of a mirror with vertical or horizontal lines drawn on it that way you will know the  circle it makes ,then time it against the video speed . should give you the release speeed exactly .

quite old thread
https://www.youtube.com/watch?v=m4tX1kCz-rY

A lot of modern cellphones can do 240 fps or faster video now. I actually have an old pocket camera that records at up to 1000fps at really low resolution. Maybe I’ll try to capture a tennis ball with it if I can make the time (but it could take a while... I already owe a video on my weighted sling!)

Tennis ball launched with asymmetric pouch: 2,655 RPM (5 rotations over 113 frames at 1000 fps while in view of the camera.
I’ll post 1000 fps video soon.
This is only one throw. Needs more trials, and it would be interesting to try with multiple slings to compare.

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A few observations:
1. A tennis ball’s inertia is very different from the rocks Sarosh threw.
2. An asymmetric sling is specifically designed to flop out of the way so the spin rate should be lower than a symmetrical pouch (all else being equal).
3. I was throwing in my yard casually... I could have thrown much harder.

Here’s the video

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I just recorded 2 rotations in 10 frames at 240 frames per second. This is 2880 RPM ± 320.  This was with a rusty ball. So all pretty consistent, around 3000 rpm. Seems to be mostly dependent on the geometry of the projectile/sling combo and how much the sling wraps around the projectile as it releases.