Tangled Physics: Knots Strain Intuitive Physical Reasoning | Hacker News Reader

Tangled Physics: Knots Strain Intuitive Physical Reasoning

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40 pointsPaulHoule10 comments

I made an animation comparing the reef and thief knots that maybe makes it a little more intuitive why one is stronger: https://x.com/KangarooPhysics/status/1526265820220952582

As a surgeon who deals in knots all day long, I can tell that I have to manually and systematically keep track of the ends at all times. Any little slip can turn the secure "surgeons knot" into an unsecure "slipknot". Intuition just doesn't seem to work here.

As someone who did a surgical rotation in med school (though ultimately decided against surgery), I can support that surgeons really are good at paying attention to their knots. I had a good number of my knots cut out. Good practice though.

It’s notable that the prior experiments described have focused on dynamics while the strength of a knot is a question of statics. It would be interesting to see further research into the intuitive understanding of other types of static systems, such as bridges(1) or various cantilever structures, and especially other systems where the dominant forces stem from static friction like in knots.

(1) Based on some very spotty anecdotal data, I wouldn’t be surprised if many people don’t even consider the primary load-bearing structures of many bridge types as crucial to the bridge’s strength but more like purely aesthetical additions.

Is it so clear that it's a question of statics? Knots are often tied in dynamic rope (static rope will lose much more strength around a tight bend radius) and if you look at break-testing youtubers like HowNot2 the knots move a lot while they are being broken. A knot that's taken to almost breaking loads looks very different to the original knot in most cases.

My impression is that a common failure mode is the rope cutting through itself, which comes down to dynamic friction and the resulting heat I think.

Looking at the graphics and knowing the answers, and having limited experience tying knots in serious situations, I still don't intuit why one knot is more secure than another. I have no idea of the principles:

* Increased friction, I assume, is the fundamental difference. Is that correct? That still leaves the question of what, in practice, contributes to friction significantly, what is inconsequential, what works or fails in combination, etc.

* How much friction do different knot components have? Can we determine the strength of a knot by simply adding the friction in different components?

* What effect does compression of the cordage have?

* How much does effect does the cordage material's natural friction have?

* What does failure really depend on, in practice? Powerful traction forces? Lubrication? Back and forth movement (e.g., that loosens the knot)?

* Can 'give' make a significant difference, in either the knot's form or in the material? As in all solid things, a knot with more give should apply less force to failing parts.

* Is distribution of forces important? A knot that distributes powerful forces to multiple spots should perform better, theoretically.

...

I don't have any answers for you but as an avid climber - you also want to be taking into account the material you are tying knots in. Super static material like dyneema really doesn't like being bent at a tight angle and will lose a lot more strength in a knot than something like nylon. Also thicker material tends to lose less strength (bigger bend radius).

But yeah... beyond that my usual method of understanding a knot is to find the places where one part of the knot constricts another under load, because that generates the friction. For instance the reef knot in the linked article has two collars that bite on two strands of the rope each when pulled, so lots of friction there and it makes sense to me it's very syrong. On the other hand I can't work out how the granny knot is stronger than the thief! Very subtle stuff.

Number of crossings tends to help (by spreading the friction around, though still not evenly). And I think people don't tend to consider just how much having any knot will weaken a rope - the basic overhand more than halves the strength[1].

1: https://igkt-solent.co.uk/knot-strength-chart/

In answer to my own question, I found this:

https://news.ycombinator.com/item?id=42523884

It keeps saying that people tie shoelaces every day so should be familiar....except most people probably do it wrong and have never questioned it. https://lifehacker.com/some-of-you-are-tying-your-shoes-wron...

I still have to remind myself to actually do half of the knot in other orientation instead of relying on muscle memory, to get the correct one.

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