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General >> General Slinging Discussion >> New vid of energy and speed of 115 g sling bullets.
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Message started by Foresight17 on Jul 29th, 2022 at 10:31am

Title: New vid of energy and speed of 115 g sling bullets.
Post by Foresight17 on Jul 29th, 2022 at 10:31am
Here's the link if interested. The description has the info, but I'll post it too. The formulas for the energy calculations will be posted soon.

https://youtu.be/MWClcGXqnN4

115 gram lead sling bullet

Momentum=4.79 kg×m/s

Velocity  = 41.71 m/s

Ft lbs of energy = 74

Area=.8"

Length=2.9"

Pressure=92.5 psi


Title: Re: New vid of energy and speed of 115 g sling bullets.
Post by joe_meadmaker on Jul 29th, 2022 at 9:14pm
Cool man.  Are you measuring the speed with some kind of device, or is it just calculated based on the distance and time?

Title: Re: New vid of energy and speed of 115 g sling bullets.
Post by IronGoober on Jul 31st, 2022 at 11:11pm
Pressure?  How is this calculated and to what is it referring?

Title: Re: New vid of energy and speed of 115 g sling bullets.
Post by Foresight17 on Aug 1st, 2022 at 10:21am

joe_meadmaker wrote on Jul 29th, 2022 at 9:14pm:
Cool man.  Are you measuring the speed with some kind of device, or is it just calculated based on the distance and time?


Heya! I took a measuring tape and measure out the distance, then fire from behind the line (in the vid, I am 21.5 meters away from the wall of sand it impacted), then just edited the video into individual frames and counted the frames from release of my hand to impact on the wall and got the measurement. It's not accurate to the foot per second, but it does get u the meter per second. I plan to buy a chronograph with detachable diffusers so I can fire over it and get exact readings.

Title: Re: New vid of energy and speed of 115 g sling bullets.
Post by Foresight17 on Aug 1st, 2022 at 10:37am

IronGoober wrote on Jul 31st, 2022 at 11:11pm:
Pressure?  How is this calculated and to what is it referring?


Pressure is a direct reference to an objects penetration qualities in correlation to the projectiles widest point. The formula to calculate this is as follows:
Pressure=Force/Area.

Example A: 35.67 gram Arrow (550 grains) from 40# recurve bow at 30 inch draw

Velocity=145 FPS
Mass=35.67 g
Area=.309 (of an inch)
KE=26Ft Lb

P=26/.309
P=84.14 PSI

The arrows total KE is divided by the arrows widest penetrating point, the shaft.

Now lets apply this to a 1 inch ball bearing

66.8 gram steel ball
Velocity=150FPS
Mass= 66.8gram
Area=1.00
KE=52FtLb
P=52/1.0
P=52PSI

Momentum is also very important but I'll be posting an in depth explanation later, however based off this we can see that at each projectiles widest point, despite the ball having way more energy, it only has a little more than half the psi of the arrow, even excluding the pressure of its tip. The ball is 1 inch all around so has no better penetrating point than anywhere else on it.

Title: Re: New vid of energy and speed of 115 g sling bullets.
Post by joe_meadmaker on Aug 1st, 2022 at 11:56am

Foresight17 wrote on Aug 1st, 2022 at 10:21am:
I took a measuring tape and measure out the distance, then fire from behind the line (in the vid, I am 21.5 meters away from the wall of sand it impacted), then just edited the video into individual frames and counted the frames from release of my hand to impact on the wall and got the measurement.

Yep, I've used the exact same method.  It's a pretty easy way to get an estimate when needed.

Title: Re: New vid of energy and speed of 115 g sling bullets.
Post by IronGoober on Aug 1st, 2022 at 4:21pm
Ok, but the values you are using are energy/area. In the calculation you just gave, you would have (ftlb/in^2) instead of psi. It is not quite the same thing.

Energy/area is still useful for ballistic calculations, but not the same concept as pressure. I just want to make sure we are all on the same page.


To calculate a value for pressure, you would need to know the rate at which the projectile decelerates (giving you the force, F=ma) and only if the deceleration was constant would you get a constant pressure value (which is not usually what happens in a real situation).  You can make assumptions about deceleration though, like assuming that it happens in a single camera frame, or something like that (1/30th of a second). As long as you are consistent with the assumption, it should give you something useful to compare different throws/ammo.

Title: Re: New vid of energy and speed of 115 g sling bullets.
Post by Foresight17 on Aug 4th, 2022 at 11:21pm

IronGoober wrote on Aug 1st, 2022 at 4:21pm:
Ok, but the values you are using are energy/area. In the calculation you just gave, you would have (ftlb/in^2) instead of psi. It is not quite the same thing.

Energy/area is still useful for ballistic calculations, but not the same concept as pressure. I just want to make sure we are all on the same page.


To calculate a value for pressure, you would need to know the rate at which the projectile decelerates (giving you the force, F=ma) and only if the deceleration was constant would you get a constant pressure value (which is not usually what happens in a real situation).  You can make assumptions about deceleration though, like assuming that it happens in a single camera frame, or something like that (1/30th of a second). As long as you are consistent with the assumption, it should give you something useful to compare different throws/ammo.


Oooohhh, ok thank u so much for pointing that out. I completely missed those errors and appreciate having that shown to me. I'll be fixing those in the future  ;)

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