Looking at the papers and the discussions here, I see 2 main ways of calculating the power of a sling. Foot-pounds and joules.
  While these are good for people who understand it, I frequently do demos for Boy and Cub Scouts. Does anyone know how I can tell them what the power is in terms that a common person would understand ( say like PSI, or anything else) ???
  It just makes it easier for people to understand.

Newton is the unit for force. If you hold 1kg here on earth it pulls down with nearly 10 Newton. On the moon it's less.

Feeling Joule is more problematic, but to understand the power of the sling it is probably helpful to compare it to .22 cal. rifles. I found for these an energy of 100-220J. A skillful slinger should come into that range. (For military high power ammunition I found energies up to 10,000J.)

Zwiebeltuete

Joules and Foot-pounds are measures of energy, not force, but there is an easy way to understand this. You can think of energy being either stored, like an elastic, or in terms of how fast something is moving, like a sling stone (kinetic energy). An even simpler way to think of stored (potential) energy is to imagine lifting a weight to a certain height. It takes work to get it up there right?

When we say that a glande hits with 100 ft-lbs of energy, we are really talking about a combination of how heavy the rock is and how fast it is going. Speed is more important than weight in this case. The energy stored is exactly the same as the amount that you would need to lift a 100 pound weight 1 foot high, or a 1 pound weight 100 ft, or a 5 pound weight 20 feet. Drop it from that height and you get the energy back as speed when it hits the ground. 100 ft-lbs is pretty significant, you don't want it to, say, land on your foot.

Joules are the metric way of measuring energy. It actually makes life much easier since metric doesn't confuse mass and weight like certain other systems, but in this case it is just slightly tricky, since most people "weigh" things in kilograms instead of Newtons. A Joule can be expressed as mass*acceleration*distance. If we use the lifted rock potential energy example, the acceleration in the formula is just gravity, which is close enough to 10  (~9.81). To use the same analogy, then, we can say that a 136 Joules is like a kilogram dropped from 13.6m high.

(1 foot-pound is close to 1.36 Joules)

Matthias

    ??? ???                                                                     
     Jeez, People, now you've got ME confused.                               
         I am accustomed to comparing the performance of different calibers by looking at ballistics tables, which here in the US conventionally list both kinetic energy in foot-pounds and velocity in feet-per-second, whereas in Europe, Joules and meters-per-second are used. But you are right, this only makes sense because these numbers are familiar to me.                                              
         If I might make a suggestion: maybe DISTANCE is something most people would relate to better. I have had a couple of occasions to demonstrate my sling and I would throw a rock BY HAND, and then throw another rock of similar size and weight with my sling. People will OOOO!! and AAAAHH!! when they see the rock fly twice or three times as far as a rock thrown by hand.                                           
         This is, of course, a relatively crude measure of "power" but at least it is easy for everyone to see.                          .......Dan

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Just for reference a newton is the force it takes to accelerate a 1kg mass by 1m/sec each second it is applied.

If you want to measure impact pressure in pounds per square inch, you need to know the mass and velocity of the slung stone/glande/whatever, and also its size.

i don't understand it either for that matter but all those things seem coolish Lol:D