After all these years, I have to say I still do not understand the commonly quoted rule of thumb, often attributed to Don Blair, that says "For every foot an object falls, it gains a unit of weight plus one". Sounds very straightforward. But then the example often given with it is what confuses the hell out of me. That example is: 500 lbs falling four feet = 2,500 lbs. Now, either I don't understand math or I don't understand English, but if you say that for every foot an object falls it gains a unit of weight, plus a unit of weight, to me that seems to mean that a 500 lb log falling four feet would hit rigging with an impact of 4,000 lbs (500 plus 500 times 4). I realize that is not right, but isn't that what that verbage is saying? I also realize that is a crude rule of thumb. Any input from those who do a lot of rigging?