[HN Gopher] Tangled Physics: Knots Strain Intuitive Physical Rea...
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Tangled Physics: Knots Strain Intuitive Physical Reasoning
Author : PaulHoule
Score : 32 points
Date : 2024-12-27 14:16 UTC (8 hours ago)
(HTM) web link (direct.mit.edu)
(TXT) w3m dump (direct.mit.edu)
| Jarmsy wrote:
| 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
| mayankjain89 wrote:
| 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.
| dillydogg wrote:
| 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.
| mmooss wrote:
| 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.
|
| ...
| bubblyworld wrote:
| 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.
| blacksmith_tb wrote:
| 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/
| Sharlin wrote:
| 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.
| bubblyworld wrote:
| 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.
| rini17 wrote:
| 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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