So many variables, so little time.
Start with F-MA. Ya know, acceleration is either positive or negative and whether it is positive or negative is totally a perspective issue. Let's call gravity a positive acceleration - just for fun. The amount of force that you generate is dependent on the amount of time the piece is permitted to fall before you start applying negative acceleration. LOTS of things decellerate here. The tree, the rope, the block, the friction device, the guy who's hands are on the tail of the rope. ALL ARE IMPORTANT FACTORS. I have seen rigging demonstrations a plenty where the piece is locked off and dropped. Yea, the forces get crazy! Force equals mass times acceleration - PERIOD! We can count on the mass being what the mass is unless you start working at the speed of light. The acceleration due to gravity is (as far as I know) 9.8 meters per second squared. The only thing you have anything that you can effect is the amount of time you allow the piece to fall and the distance which you try to get that back to stopped (0 velocity). From trial and error: If you use a short distance to decellerate, you are screwed. If you allow the piece to free fall a great distance, you are screwed. If you rely on a stretch rope to decellerate, you are screwed. If you rely on the tree to absorb the energy, you are screwed. It is totally a dance. PAY THE GROUND GUY MORE AND TRAIN THE HELL OUT OF HIM!!! Communicate, let the ground guy make the call whether it is ok to take the piece (thumbs up). Buy the best available, take care of and inspect the rope after EVERY use. It is easy to do - feel it on the way back to you! Feel a bump? YOU f'd up - broken rope - better cut it into little pieces for pulling with a skid steerer. Get the friction device out of the truck you lazy (#$(*#. Install a block you lazy #$%@#. Just sayin... I get aggitated when there are "accidents" when some "tree expert" doesn't understand the basics of falling bodies and vector forces. That is killing us - literally.
I am pretty adamant about my staff understanding the principles of physics. Calculating exact weights and forces involved would be painful. I like the 10% WLL, just remember that the forces acting on the "system" are quite capable of taking your mass of 150 lbs and giving a realized force that is way, way, way past the 10% WLL. If you are using a porta wrap or a friction device - limit your wraps to 2 and a half! After that number of wraps, you will be locking the force into your rigging system and guess what - YOU ARE SCREWED. Maybe not today with that brand new rope, but you will be screwed eventually and I assure you it will not be with lube.
