dbldrew, I think you're leaving something out from your own example. It is saying the impact force is 106 kN, so if the calculator is junk so is your example. You keep citing the energy as impact force. When that energy is distributed over a very short distance you end up with some pretty large impact forces. Honestly I can't sit here and say I completely understand impact force, but everything I've found on this points out these high impact forces including your example.
For the record, since the original post I've switched from the ropeman to a blakes or prusik. I use my tether while climbing, so I do end up with some slack at times and don't feel comfortable with the ropeman in that scenario. I agree that if you're not using it while climbing and just hooking your tether up at height then it's highly unlikely you would have issues with the ropeman. I also switched to a dynamic rope to further lesson the impact should I happen to fall a few feet. As a climber you can probably see how that would make a pretty big difference.
I know you and Matt are going back and forth here pretty hard here, but he's fighting the good fight. Most of what he's saying is what I've found after researching this pretty hard. I would have to say for as much as we talk safety on here there isn't a very good understanding of impact force. IMO, it's not enough to say well if it's good enough for climbers it's good enough for me. Without understanding what we're doing, we could be subjecting ourselves to greater forces than a climber would.
Keep going in your example to see that they are coming up with 106kN for the impact force.
Example - a Person falling from a Table
A person with
weight (gravitational force) of
200 lbs (lbf) falls from a
4 feet high table.
The energy of the falling body when it hits the ground can be calculated using
(4) as
E = (200 lbf) (4 ft)
= 800 ft lb
The impact on a human body can be difficult to determine since it depends on how the body hits the ground - which part of the body, the angle of the body and/or if hands are used to protect the body and so on.
For this example we use an impact distance of
3/4 inch (0.0625 ft) to calculate the impact force:
Fmax = 2 (800 ft lb) / (0.0625 ft)
= 25600 lbf
In metric units - person with weight
90 kg, falling distance
1.2 m and impact distance
2 cm:
E = (90 kg) (9.81 m/s2) (1.2 m)
= 1059 J
Fmax = 2 (1059 J) / (0.02 m)
= 106 kN