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Would a hemp rope with a 30mm diameter break if a 100 kg object is attached to one end and dropped from a bridge (free-fall). The other end of the rope is attached to a pole on the bridge. The object would fall 4 meters, after that the rope would stretch completely. Bridge height is 6 meters, so after the 4 meter fall from the top of the bridge the object would still be in the air.

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    Why do you need to know? I'm asking because if it doesn't break, hemp has got very little elasticity, and the 100 kg object might be harmed by the force generated. Jan 25, 2023 at 11:39
  • @TobySpeight just curious about the strength of a hemp rope, that 100kg object is irrelevant, could be a boulder.
    – a a
    Jan 25, 2023 at 11:42
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    It may well depend on how the rope is made, some ways of making rope result in stronger others in more elastic ropes.
    – Willeke
    Jan 25, 2023 at 15:08

1 Answer 1

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TLDR: YGD

It all depends and there are many variables that we don't know exactly like the earlier comments mentioned, but it sounded interesting to try an estimate. And thanks in advance for any corrections to my math (its been 40 years since I took this class).

The velocity at 4 meters of free fall can be found from:

v² = u² + 2as

where s = displacement; u = initial velocity; v = final velocity; a = acceleration;

v² = 0 + 2x9.81m/s^2x4m

v=8.9m/s

This link provides the stopping force, I made a couple guesses on rope stretch and got the following

0.01 m.. 396 KN

0.1 m .. 40 KN

This link manilla rope strength provides what I would think of as an estimate of magnitude for the rope strength where it lists the Minimum Breaking Strength of 30mm at 54 KN.

So depending on how much stretch you get, rope breakage is possible, and there is a good chance that some other part of your anchoring or attachments will break.

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  • Sorry but manilla and hemp are very different in break strength and stretch. Maybe you can find a break table for hemp.
    – Willeke
    Jan 26, 2023 at 19:30
  • The terms manilla and hemp have been used interchangeably, but in either case, how the rope is made and how much stretch it has could easily change the numbers by an order of magnitude. It would be silly to think that any data you find out there applies to the rope you have in your hand. I doubt there is any industrial trade group out there that certifies hemp rope strength. As I said in the answer, this just gives an estimate of the order of magnitude.
    – Mike
    Jan 26, 2023 at 21:31
  • That is like saying that a polyester rope and a polyethyleen rope are the same, as both are called poly ropes. You show a better understanding in your answer.
    – Willeke
    Jan 27, 2023 at 5:56

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