Tying knots in rope(s) - tension, weakened strength

[trishonutila]

Hi all,

Found this forum while looking for some resources for my students...my short question is: does anyone know of a simple explanation of the physics involved in tying knots in rope?

Background is - I am a chemical engineer by degree, but I recently taught high school physics for a few years, and now I am teaching physics as part of a "science" course for 7th graders, so I need to keep it simple. Because I am teaching in Utila, a small island off the coast of Honduras where fishing and sailing have been primary livelihoods for generations, I decided to include knot-tying in with physics to help keep their interest and just in case they decide to go into the traditional jobs.

I have some nice information on tying the knots, but in the course of that search, I found many sites that claimed that each knot "weakens" the rope, but no real explanation as to why that is. I can handle explaining "tension" to the level that my students are at, but I am a little short on why one knot is "better" than another for strength (other than statements made on sites describing the knots) and I am at a loss for the "weakened strength" associated with each knot tied idea. Hence, my question above. Alternately, if someone here has a suggestion for a simple explanation, I would greatly appreciate it. (I searched through some of the threads before I decided to post and found many of your answers to other questions to be exactly that - Thanks!)

 

[Redbelly98]

Admin note: thread moved to General Physics

 

[rcgldr]

My guess is that it's related to how much "bend" there is in at the ends of the knot. For radio control gliders, the most common knot used in the fishing line used to launch the models (there's also a long length of latex tubing use to generate tension), is the "uni knot", although it's the worst in terms of "bend", being 180 degrees on the loop end, but 0 degrees on the other end. However the loop end is normally wrapped around a ring or stake, which reduces the "bend".

http://www.fish4fun.com/Lines_to_Tackle.htm

 

[nucleus]

Adel’s Millwrights and Mechanics Guide says:
“ A rope is weakened by knots. In order to form a knot, the rope must be bent, which brings most of the strain on the outside fibers. The overloading breaks the outside fibers, increasing the strain on the fibers below, which later break and soon the entire rope breaks. From experiments, the approximate efficiency of knots, hitches, and splices varies as follows: straight rope, 100%; eye splice over an iron eye, 90%; short splice, 80%; timber hitch anchor bend, 65%; clove hitch running bowline, 60%; overhand knot, 45%.”

The results of how much different knots weaken a rope are available here:
http://blank005.tripod.com/camping/howtoknots.html