Hello slingers.
Have anybody found out how much energy a slingshot will give? Mesuring in ft.lb or kgm.
I am thinking of a normal throw using both roman glans and a stone.
Does anybody know this?

Sorry.
My bad spelling. Sling it is

Here are some figures I came up with.  I'll go through the calculations in a minute, but the bottom line is: a well-slung rock can pack nearly the same punch as a popular military side-arm.

OK, here's the details:

Speed of rock at release equals rotational speed of sling.  That, in turn, is circumference of circle the sling swings in, divided by how long it takes for a full circle.  Circumference is twice the radius, times pi.  Radius is my arm length plus the sling length; call it 5 feet  or 1.5 meter.  Rotational speed of sling, just before release, somewhere between 7 and 10 revs/second.  Pi is 3.14 (plus a few million more digits that we won't worry about).

Putting the above data together, we can get minimum and maximum figures for the speed the rock is moving as it leaves my sling:
   2 * 3.14 * 5 / (1/7)    = 220 ft/sec = 150 mi/hr
   2 * 3.14 * 5 / (1/10)  = 314 ft/sec = 214 mi/hr

In metric, the speed range is  to 67.0 to 95.8 m/s (241 to 345 km/hr).

Now, the kinetic energy of the rock is found by multiplying half its mass by the square of its speed.  If the rock I'm slinging masses 4 ounces (113 grams)...

"Too confusing for English, switching to metric!"

OK, with the mass in kilograms and the speed in meters/second,
   0.5 * 0.113 * 67.0² = 254 joules
   0.5 * 0.113 * 95.8² = 519 joules

Converting this back to English units, that's in the range of 187 to 382 foot-pounds.

For comparison, the kinetic energy of a .45 caliber bullet fired from a Colt automatic (standard U.S. Army sidearm, or used to be) is around 500 to 550 joules (369 to 405 foot-pounds) at the muzzle (depends on who measured it, I saw different figures in different tables).

I think somebody like Jurek, or Knollslinger, or Larry Bray, would probably exceed the 10 revs/second figure, so their rocks would pack even more punch.  I was going to measure Jurek's speed when he knocked a tire off a wall, but I cannot view his video anymore ... lost the video codec or something.

Hmmm... Roman military physicians had an tool practically identical to what modern physicians use to remove bullets.  The Romans used this tool to remove glandes.  Now we know why they needed such a tool.

Be careful with those slings, people!