Rope / String Strength Question

[Swarley]

I have no idea how to google this.

I have some cotton that can hold 2lb when tied with a granny knot.

I tie one end to a hook on a plank of wood, and the other to a fishing scale. I pull it back and when it gets to 2lb it snaps. I can understand this, my cotton has the tensile strength of 2lb when knotted to a hook by a granny knot.



Now this is my question.
I granny knot my 2lb cotton to the hook, then pass the cotton through the ring in the scale then tie it back
on to the hook. Then pull it till it snaps.
But this time it snaps at 3lb. Why?
Why can this configuration have a higher tensile strength, when at one point (the ring on the scale) it is only one strand of cotton? Sherly its weakest link is at the ring?

[Swarley]

Not homework. Its been a question that's been bugging me for years.

And yes I did have some spare time today.

[Swarley]

Twice at the ring, once half way down the cotton on one side.

[Swarley]

dilbert said:

Swarley said:

Twice at the ring, once half way down the cotton on one side.

But you don't say which configuration did what....

Oh sorry
Twice at the ring, once half way down the cotton on one side, for the second test.

At a knot every time with test one.

[Swarley]

mblade123 said:

Your halfing the loading. Each length of string now takes half the load in the double configuration.
Although I do wonder why it snaps at 3Lbs and not 4.
If you had 2 more scales at the "grany knot end" you would find each should register 1.5lbs

I agree. But at one point (the ring) there is only one strand of cotton that can only hold 2lb. So how can it get up to 3lb at that point?

[Swarley]

Oh and will it take off?

[Swarley]

mblade123 said:

Swarley said:

mblade123 said:

Your halfing the loading. Each length of string now takes half the load in the double configuration.
Although I do wonder why it snaps at 3Lbs and not 4.
If you had 2 more scales at the "grany knot end" you would find each should register 1.5lbs

I agree. But at one point (the ring) there is only one strand of cotton that can only hold 2lb. So how can it get up to 3lb at that point?

You would think so but thats not the case. Infact you are still doubling the string. If you had a roller in there instead of the ring it would be different again as you are increasing the MBR. ( Mininmum Bend Radius)

Ok then, So in that case my cotton could have a tensile strength of 3Lbs, but my knots reduce this to 2lbs. Then at the ring my cotton is working as if it was perfectly straight (ie capable of holding 3Lbs), and the 2 knots are working together (ie 1.3Lbs each).
So I would not have a week link, only slight differences in the cotton itself, this would mean it would never or be unlikely to snap in the same place. It doesn't so you must be right!

Thank you!!!!

[Swarley]

mblade123 said:

CyprusCraig said:

Reently done a banksman slinging course?? haha

Also depending on the angle of the lift aswell.

Correct the angle is all important lol.

Stage 3 North sea lifting course.
Advanced Rigging and lifting.
Deck Foreman on heavy construction vessels.
LOL

I defiantly found the right person for my problem then!