Welcome, Guest. Please Login
SLINGING.ORG
 
Home Help Search Login


Page Index Toggle Pages: 1
Send Topic Print
Spin rate of a stone? (Read 1945 times)
Sarosh
Interfector Viris Spurii
*****
Offline



Posts: 1136
Spin rate of a stone?
Nov 7th, 2018 at 3:43pm
 
quick question :
has there been any measurement of spin rate of a thrown projectile from a sling?

also how would you do it?
Back to top
 
WWW  
IP Logged
 
Tint
Interfector Viris Spurii
Past Moderator
*****
Offline



Posts: 2323
Hong Kong
Gender: male
Re: Spin rate of a stone?
Reply #1 - Nov 7th, 2018 at 11:28pm
 
Very interesting question!

It would take a very expensive set up with slow motion cameras to measure the spin of the rocks accurately.

In tennis training, players are taught to watch the seams of the ball in mid flight to judge if their shots are spinning fast enough. We know that movies needs to be at least 24 frames per second for the images to look smooth. So if the ball spins so fast that the seams are not visible, we assume it is safe to say that it is spinning at over 1,000 rotation per minute and that's enough to create some "kick" to the bounce and curve the flight of the ball to affect the game even at very high level.

I think it is much harder to spin a rock that fast. But even so, I have had rocks that curve in peculiar ways in mid flight. Sometimes the spin of the rocks is even audible! I think that may have something to do with the irregular shapes of the projectiles.
Back to top
 
WWW  
IP Logged
 
Sarosh
Interfector Viris Spurii
*****
Offline



Posts: 1136
Re: Spin rate of a stone?
Reply #2 - Nov 8th, 2018 at 3:38am
 
https://www.youtube.com/watch?v=6GWL8Gt-BsQ
watch from 9:46

@ 10:19 he says up to 3000 rpm
and the same is said for baseball
i would expect that a sling stone has at least 2000rpm
but  it would be nice to know if and how the rpm changes with different styles, stone diameters, densities.
how much energy is spent to rotate the projectile? energy that may be better used as kinetic energy...
Back to top
 
WWW  
IP Logged
 
Teg
Interfector Viris Spurii
*****
Offline



Posts: 1113
Central Europe
Gender: male
Re: Spin rate of a stone?
Reply #3 - Nov 8th, 2018 at 7:37am
 
Tint wrote on Nov 7th, 2018 at 11:28pm:
Sometimes the spin of the rocks is even audible!


If you want to go cheap, start there. I would record the sound of a buzzing stone with a microphone, then do a frequency analysis (Fourier transform or STFT if needed). Depending on how the sound looks, the frequency spectrum could be enough. Otherwise you could look for amplitude modulations at certain frequencies. Most probably a correction for the doppler effect will be needed.  The frequency of the perceived sound will then be a measurement of the rotation frequency.

Cheap on the experimental side, a bit more expensive in time and knowledge required for the evaluation  Wink

For a camera: 3000 rpm is 60 rps or Hz. You want to sample at least twice as fast (i.e. 120 Hz). to have some margin, go for ~1000 Hz sampling. A camera with decent resolution will cost on the order of 3000 dollar or so... (example chronos 1.4 or whatever the current iteration is). Industrial setups are ~25k+ the last time I looked. And be prepared for gigabytes of data Wink
Back to top
 
 
IP Logged
 
timpa
Interfector Viris Spurii
*****
Offline


Stones in the horizon

Posts: 1514
Finland
Re: Spin rate of a stone?
Reply #4 - Nov 8th, 2018 at 1:27pm
 
This is the question I've been thinking until the 70's.
If anyone finds out, I'm looking forward to the results.

A friction pouch is needed to prevent the stone from slipping.
Back to top
 
 
IP Logged
 
Frondeur
Junior Member
**
Offline


I'm the lead slinger in
a hard rock band

Posts: 90
France
Re: Spin rate of a stone?
Reply #5 - Nov 9th, 2018 at 6:35am
 
a bit of aproximatology:
let's assume a starting speed of 30 m/s, and a stone circumference of 10 cm, or 0.1m
IF the stone is simply rolling out of the pouch, tha would give us 300 rotations/s, or 18000 rpm Shocked

Obviously, there's a important "slipping factor" witch would severly reduce this incredible number . I guess there's an "angle factor" too, to reflect the fact that the stone may not get out in a perfect way.

Those petty considerations leads us to no answer, but show that 2 or 3000 are perfectly realistic.

As for the sound effect, consider that the audible frequences (for a human) are between 20 and 20000 Hz. that's au very wide fork, but perfectly suited for a sling stone...

Well... feel's like I'm going nowhere...

Back to top
 
 
IP Logged
 
Sarosh
Interfector Viris Spurii
*****
Offline



Posts: 1136
Re: Spin rate of a stone?
Reply #6 - Nov 9th, 2018 at 9:21am
 
Teg wrote on Nov 8th, 2018 at 7:37am:
do a frequency analysis


Thanks for the idea!
I think i'm getting somewhere with audacity. Cheesy

3 measurements i consider clear enough gave results:
150, 175 and 225 rps
that's 9000 , 10500 and 13500 rpm !

I'll check if i can find other methods with audacity and maybe i'll make a video about it (later) and then we can talk about possible errors.
Back to top
 
WWW  
IP Logged
 
Teg
Interfector Viris Spurii
*****
Offline



Posts: 1113
Central Europe
Gender: male
Re: Spin rate of a stone?
Reply #7 - Nov 9th, 2018 at 11:01am
 
Thumbs Up

Sarosh wrote on Nov 9th, 2018 at 9:21am:
and then we can talk about possible errors.

A flat stone might give two "flops" per revolution, i.e. there could be a factor 2 error in there. Not sure though, as it's not my field of expertise  Wink That's the biggest source of error I see.

Good luck!
Back to top
 
 
IP Logged
 
Drakolith
Funditor
****
Offline



Posts: 675
U.S.
Re: Spin rate of a stone?
Reply #8 - Nov 9th, 2018 at 11:14am
 
Not sure if this will help at all, and this isn’t a stone per say, but when I made some clay ammo using CAs awesome roller, I let one dry a lil then threw it at a brick wall, albeit not from an extreme distance but not exactly point blank either, and the spin is obvious due to the warping of the text and it hit point first as you can see the other point in the middle. Here’s the picture. I believe I posted this already so some may recognize it.
Back to top
 

“When you do things right, people won’t be sure you’ve done anything at all.”
 
IP Logged
 
Sarosh
Interfector Viris Spurii
*****
Offline



Posts: 1136
Re: Spin rate of a stone?
Reply #9 - Nov 9th, 2018 at 11:56am
 
Teg wrote on Nov 9th, 2018 at 11:01am:
A flat stone might give two "flops" per revolution


the stones used had 4 sides (rectangular) , 4 flops per revolution, i counted that  Wink . frequencies produced are @ 700Hz with some reaching 900Hz.

PS: i wonder what precession of the stone does to the frequencies
Back to top
 
WWW  
IP Logged
 
Teg
Interfector Viris Spurii
*****
Offline



Posts: 1113
Central Europe
Gender: male
Re: Spin rate of a stone?
Reply #10 - Nov 10th, 2018 at 6:04pm
 
Sarosh wrote on Nov 9th, 2018 at 11:56am:
PS: i wonder what precession of the stone does to the frequencies


Nothing?  Cheesy
My guess would be modulation of the amplitude of the flops with the precession frequency, i.e. the appearance of another frequency and thus a "beat".  Though I'd assume not through interference of emitted sound waves  as in the usual usage of the term "beat" (https://en.wikipedia.org/wiki/Beat_(acoustics) ) but more because of the directionality of the emitted sound waves... But this is now pure speculation  Wink.
Back to top
 
 
IP Logged
 
NooneOfConsequence
Slinging.org Moderator
*****
Offline



Posts: 2983
Texas
Re: Spin rate of a stone?
Reply #11 - Nov 10th, 2018 at 9:42pm
 
It actually wouldn’t be too hard to get an answer on video. 3000 RPM is about 50 Hz. You won’t hear that very easily with most low-cost microphones, but I bought a 1000fps Camera on Ebay for about $100.  That should be able to see up to about 30,000 RPM.  Even an iPhone can do 240fps, which is fast enough to measure over 7,000 RPM. Just paint the ammo half black and half white.

Just remember that the energy that goes into spin is energy that doesn’t make it move forward.
Back to top
 

“My final hour is at hand. We face an enemy more numerous and cunning than the world has yet seen. Remember your training, and do not fear the hordes of Judas. I, without sin, shall cast the first stone. That will be your sign to attack! But you shall not fight this unholy enemy with stones. No! RAZOR GLANDES!  Aim for the eyes! May the Lord have mercy, for we shall show none!“  -Jesus the Noodler
 
IP Logged
 
NooneOfConsequence
Slinging.org Moderator
*****
Offline



Posts: 2983
Texas
Re: Spin rate of a stone?
Reply #12 - Nov 10th, 2018 at 9:45pm
 
Perhaps this discussion belongs over in maths?  The maths topic hasn’t been given a lot of love lately.
Back to top
 

“My final hour is at hand. We face an enemy more numerous and cunning than the world has yet seen. Remember your training, and do not fear the hordes of Judas. I, without sin, shall cast the first stone. That will be your sign to attack! But you shall not fight this unholy enemy with stones. No! RAZOR GLANDES!  Aim for the eyes! May the Lord have mercy, for we shall show none!“  -Jesus the Noodler
 
IP Logged
 
Page Index Toggle Pages: 1
Send Topic Print
(Moderators: vetryan15, Kick, Rat Man, Morphy, Chris, joe_meadmaker, Curious Aardvark)