Using parachute cord to abseil down a cliff
From BearWiki
Claim: Using a parachute cord to abseil down a cliff is a dangerous idea.
Bear uses parachute cord near the end of the show to abseil down a cliff to reach a highway. Parachute cord?!? To abseil? It has a low breaking strength. Even doubling it up would barely hold your static weight. Any slip, even a short one, would add significant force. Not to mention the significant possibility of the cord fraying over rocks.
The strongest parachute cord, which is not likely the presented cord, has a breaking strength of 750 lbs (type IV). It can be as low as ~100 lbs (type I and IA). It appears to be the most common, type III ("550 cord", so called for its 550 lbs breaking strength).[1]. Most rock climbing equipment, on the other hand, has a minimum breaking strength of 4,500 lbs. [2] When climbing, it's not about supporting your weight; it's about resisting the inevitable jolts and abrasion damage, which parachute cord is not designed to tolerate.
Let's do the math, being generous and assuming the common 550 cord. Let's say that, should you fall, the cord will have a whole quarter of a second to absorb the impact (the actual length of time could be calculated with Young's modulus for nylon, which is 5e9N/m^2, but we'll leave that as an exercise to the reader). Let's say that it miraculously hasn't frayed at all. 550 lbs is ~2500 newtons. The force of impact is f = ma. Let's assume a 100 kilogram man, after gear. So, given that the force of the fall is the breaking force minus the (always present) force of gravity, we have 2500 - 100 * 9.81 N = 100 a, so a = 15.2 m/s^2. Giving that a quarter second, we get that the falling velocity can be given by v = 3.8 m/s. For the fall, d = 1/2 at^2, and v = at, so v = a * sqrt(2 d / a), which simplifies to v = 4.43 d and d = v / 4.43 using a gravitational acceleration of 9.81 m/s^2. Plugging in our known breaking velocity, we get d = 3.8 / 4.43, i.e., 0.85 meters (~2 1/2 feet) to reliably snap this undamaged 550 cord. Use a lighter parachute cord, have a faster stop speed, be carrying heavy gear, or damage the cord, and this number can be greatly reduced.
[edit] Analysis
Support: I'll take the less dramatic but safer walk down instead, thanks.
Support: The actual shot of him abseiling doesn't show the rope wrapping around his body. The rope passes on his right, in front of his body into what is probably a figure eight or ATC (not visible) and then vertically above his head. The image isn't perfectly clear, but it is enough to suggest that the rope doesn't wrap around his body as the pre-descent image shows.
Support: Unless you're wearing padding, body rappels are painful, even with 11mm rope. Using parachute cord without something like leather padding would cause injury, even if the cord didn't break.
