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force on a stone, slinging experiences (Read 3082 times)
Dravonk
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force on a stone, slinging experiences
Mar 10th, 2007 at 3:56pm
 
Today I was slinging stones at a lake again. The water level is quite high now, most of the beach is gone. Time to look for another source of many stones.

When I was slinging stones I sometimes noticed two stones leaving my pouch although I only put one stone in. But today something very strange happend. I took a really big stone, threw it and after half the flight it split into two (in midair!). Now I wonder what caused that. Is it the air friction, is it the rotation?
Oh yes, I do need to find better stones...maybe the rough stones are one reason for my poor range.

One shot gave me a fright. I fired a rather big stone when I suddenly saw a duck flying very low over the water and it looked like my stone might hit. The stone hit the water very close to the duck...and the duck plunged into the water too. Shocked Luckily it got up again and continued its flight. I guess I frightend it so badly it forgot to fly and plunged down.

On my way back two deers crossed my way. They can be lucky I was on a bike and not in car.
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slingwizzy
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Re: force on a stone, slinging experiences
Reply #1 - Mar 10th, 2007 at 4:27pm
 
maye the big stone that broke in mid-air, was already broken, but didnt split because it was held by clay or something?
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wanderer
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Re: force on a stone, slinging experiences
Reply #2 - Mar 11th, 2007 at 3:18am
 
slingwizzy wrote on Mar 10th, 2007 at 4:27pm:
maye the big stone that broke in mid-air, was already broken, but didnt split because it was held by clay or something?


Seems likely. I think the rotational effects are more likely to blame than just air resistance.
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Re: force on a stone, slinging experiences
Reply #3 - Mar 11th, 2007 at 4:19am
 
Yes, I also think it unlikely the stone broke due to the centrifugal force. Remember, you are holding the other end of the cords! The force on the stone is the same as the pull you are feeling from the sling as it circles around your head. If not then Mr. Newton got his laws of motion wrong. Strictly speaking there is no such thing as centrifugal force only centripetal; it is you who exerts the force on the stone to alter its path from straight line into the circular motion of the sling. The tension you feel in the cords as you rotate the sling is the reaction to the force your arm supplies. Slings don't create something from nothing. The sling just makes the force you supply much more efficient than throwing by hand. Don't worry if you don't get this. It's high school physics, pretty theoretical.

On the other hand rough or badly shaped stones eat up that wonderful efficiency that your sling just provided. As an experiment, try slinging a golf ball and a really rough, oddly shaped stone of approximately the same size/weight, one after the other. See the difference in performance! Not only accuracy but speed and distance is dramatically reduced. Stones that make a whirring sound as they fly are particularly poor performers and the faster you sling the greater this inefficiency becomes. That is why this effect on efficiency is not so evident when you merely throw stones; the velocity is too low to really highlight it.

As in archery, where it is just as important to have good arrows as a good bow, in slinging to get good performance you need good ammunition.


Aussieslinger
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wanderer
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Re: force on a stone, slinging experiences
Reply #4 - Mar 11th, 2007 at 4:49am
 
Well, I did think about this a little more..

The stones can be spinning pretty fast when they leave the sling. If the pitch of them corresponds to their rotational speed they may be doing about 100 revs per second. That gives a centripetal acceleration at a distance of 1cm of 4.103m/s2, or about 400 times gravity. Seems to me that might be enough to pull things apart that we thought were quite well held together Smiley
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Dravonk
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Re: force on a stone, slinging experiences
Reply #5 - Mar 11th, 2007 at 5:00am
 
Yes I think the stones were already damaged and just stuck together on a very weak connection. I was just a bit confused that this connection broke up in midair rather than in the pouch. In midair I thought there were almost no forces on the stone.

Aussie wrote on Mar 11th, 2007 at 4:19am:
Yes, I also think it unlikely the stone broke due to the centrifugal force. Remember, you are holding the other end of the cords! The force on the stone is the same as the pull you are feeling from the sling as it circles around your head. If not then Mr. Newton got his laws of motion wrong. Strictly speaking there is no such thing as centrifugal force only centripetal; it is you who exerts the force on the stone to alter its path from straight line into the circular motion of the sling. The tension you feel in the cords as you rotate the sling is the reaction to the force your arm supplies. Slings don't create something from nothing. The sling just makes the force you supply much more efficient than throwing by hand. Don't worry if you don't get this. It's high school physics, pretty theoretical.

I think you need to divide the energy of the stone before the release in two parts. There is a tangential component and you do not feel that as it is perpendicular to your sling. You only feel the radial component (the longer your sling, the smaller it is: FZ=mv2/r). I know that you need to put all the energy in it yourself, the sling just adds leverage and acts as a short-time energy storage.

My question was more to what happens to the stone after the release. Maybe my question would be clearer if I had asked "was it the spin?" (of the stone) rather than "was it the rotation?".

I hope I didn't say anything stupid and the "high school physics" was not aimed at me. I study electro engineering and even though I do not have to deal with mechanics often I hope I still know the basics. Wink
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Willeke
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Re: force on a stone, slinging experiences
Reply #6 - Mar 11th, 2007 at 5:45am
 
Some of us are lucky to find any stones at all, so we sling what is there, or balls we bring in and loose all the time.
But good to know what forces we are playing with.

Willeke
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Re: force on a stone, slinging experiences
Reply #7 - Mar 11th, 2007 at 1:59pm
 
I sling on the faimly ranch in central Oklahoma. My father, the ever fastidious greenskeeper he is, lets me know when he finds " River rock" in odd or strange places like in the middle of a pasture Smiley
Lately he told me he is finding rock fragments. I thought that ment the rocks were breaking up on impact with the clay based soil we have around here.  But I guess it's possible that they are breaking up in flight. I am normally tossing underhand for distance so i don't really get a chance to see what's happening during a large point of the trajectory.

Marc Adkins

Dravonk wrote on Mar 10th, 2007 at 3:56pm:
Today I was slinging stones at a lake again. The water level is quite high now, most of the beach is gone. Time to look for another source of many stones.

When I was slinging stones I sometimes noticed two stones leaving my pouch although I only put one stone in. But today something very strange happend. I took a really big stone, threw it and after half the flight it split into two (in midair!). Now I wonder what caused that. Is it the air friction, is it the rotation?
Oh yes, I do need to find better stones...maybe the rough stones are one reason for my poor range.

One shot gave me a fright. I fired a rather big stone when I suddenly saw a duck flying very low over the water and it looked like my stone might hit. The stone hit the water very close to the duck...and the duck plunged into the water too. Shocked Luckily it got up again and continued its flight. I guess I frightend it so badly it forgot to fly and plunged down.

On my way back two deers crossed my way. They can be lucky I was on a bike and not in car.

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« Last Edit: Mar 15th, 2007 at 11:15am by winkleried »  

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Re: force on a stone, slinging experiences
Reply #8 - Mar 12th, 2007 at 5:28am
 
Hi All,

My apologies if I came across as a bombastic ass. The level of people's background knowledge varies so much and some people are totally bewildered by such notions as centripetal force and angular acceleration. I once had a fairly long conversation with a fully qualified electrical engineer who did not understand that all the particles of a wheel in circular motion were undergoing acceleration even if the peripheral speed of the wheel was constant. Some time ago Yurek posted a vector diagram showing all the forces acting on the sling during the wind up stage. Actually the hand leads the pouch and is not in the centre of the circle.

On reflection I think that both Wanderer and Slingwizzy are correct. When a stone breaks apart on leaving the pouch it is probably the spin imparted that breaks it up if there is a flaw in it already. Funnily enough I had that very thing happen to me this morning. I didn't really examine the particular stone though and no other stones flew apart. Also they were all taken from a stream. Perhaps water fills the small imperfections in the stones and they look sound but are ready to break apart when subjected to the violence of the release.

A question for Wanderer. What percentage of the stone's total kinetic energy is due to the rotation about its own axis compared with the KE due to linear motion? Also if it is possible to stop this rotation would that mean the stone would now fly faster and consequently further.

I rarely sling stones even though I often cycle along a path next to a railway line. The path is littered with stones just waiting to be picked up, but they are so angular and sharp that slinging them is just a source of frustration not pleasure. One of the reasons I usually sling balls, apart from their better performance, is that I can usually recover them and get lots of use out of one. Golf balls are almost indestructible and if you get the fluorescent yellow or pink ones they are usually fairly easy to find. Just be careful they don't bounce back at you.

Willeke,

Perhaps you could make some clay glands for those special occassions when you need extra accuracy. Also there is an article in the main section about making single use projectiles from concrete. I have not tried it myself but it sounds good. I did try filling cheap practise golf balls with cement. These flew well, but once they hit something hard the concrete just pulverised.

Regards,

Aussieslinger
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wanderer
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Re: force on a stone, slinging experiences
Reply #9 - Mar 13th, 2007 at 6:44am
 
Aussie wrote on Mar 12th, 2007 at 5:28am:
Hi All,

My apologies if I came across as a bombastic ass. The level of people's background knowledge varies so much ..........

I didn't take it that way. Smiley -
Quote:
A question for Wanderer. What percentage of the stone's total kinetic energy is due to the rotation about its own axis compared with the KE due to linear motion? Also if it is possible to stop this rotation would that mean the stone would now fly faster and consequently further.

I see another can of worms opening here.
To cut a long story short Cheesy, I think it's possible that something up to 50% could go into the rotational motion, but in practice probably 30-40% is the practical 'upper bound'. That would need a grippy pouch and or irregular shaped stones (mine are all thickly twined woven pockets which are very grippy). I suppose the closest school physics to hijack is a ball rolling down an inclined plane, where more linear velocity is available if it doesn't roll (I'm also imagining a non-spinning ball skipping off a table, which is bound to slow down if it has to start rolling) - but then ..... I can write down my arguments but it doesn't seem to me this is the place to put them. Actually almost nothing I have written above is for me unalterable 'truth' - it's my current belief about the thing!

The sling gives a lot to think about. But we all knew that Smiley - and when it gets too frustrating to think about we can go out and smash things with it;D
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Re: force on a stone, slinging experiences
Reply #10 - Mar 13th, 2007 at 11:49am
 
Aussie wrote on Mar 12th, 2007 at 5:28am:
My apologies if I came across as a bombastic ass.

Don't worry I didn't feel insulted, I just wondered whether I said anything that was wrong and sounded like I didn't take physics in school.

Aussie wrote on Mar 12th, 2007 at 5:28am:
Some time ago Yurek posted a vector diagram showing all the forces acting on the sling during the wind up stage. Actually the hand leads the pouch and is not in the centre of the circle.

True, but it depends on your coordinate system. When it is stationary to the ground you are right. But as I was discussing the forces on the grip I thought of a coordinate system that is moving with the hand. And then the hand is always in the center. *nitpick* Wink
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Re: force on a stone, slinging experiences
Reply #11 - Mar 13th, 2007 at 12:38pm
 
the duck was probably after the fish it thought had just splashed beneath it, ducks in flight don't scare easy, but they will go after an easy meal :-)

The question nobody seems to have asked (unless I mised it in all thoses repititive quotes (:-) )
What kind of stone are you using ?
Shale, soft sandstone or slate will split up with very little effort applied so would be very likely to split after leaving a sling pouch where it's just been held quite tight and then spun.

A lump of granite will never break up on leaving a sling pouch. So it's far more likely to be down to the lack of cohesion in the stones structure than any massive forces exerted on it by you :-)
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Re: force on a stone, slinging experiences
Reply #12 - Mar 13th, 2007 at 4:01pm
 
well the truth is why ever it happens It happens I jhave expernced this many times.
just more proof to the power of the sling
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Re: force on a stone, slinging experiences
Reply #13 - Mar 13th, 2007 at 4:21pm
 
Yes, the stones that broke were rather big and weak. Still I did not expect the stones to break. But breaking in a pouch is one thing, breaking during the flight (as it happend to me only once) is another thing and that made me wonder what forces could break a (weak) stone during the flight.
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Re: force on a stone, slinging experiences
Reply #14 - Mar 13th, 2007 at 7:27pm
 
[quote author=curious_aardvark link=1173560209/0#11 date=1173803937]
Shale, soft sandstone or slate will split up with very little effort applied so would be very likely to split after leaving a sling pouch where it's just been held quite tight and then spun.

A lump of granite will never break up on leaving a sling pouch. So it's far more likely to be down to the lack of cohesion in the stones structure than any massive forces exerted on it by you :-)
[/quote]
Don't disagree with any of that. ;). 400 seems a big number, but given the fairly small weights of stones, the forces are the same magnitude as you would be able to exert by hand.
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