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Topic Date User

Rope - static v dynamic for top-roping and leading 30-Jan-2012 At 11:12:30 PM Olbert

Message

On 30/01/2012 Dan Theman wrote:
>>The peak level of force is less with a dynamic rope than a static because
>>of the stretch.
>
>So in other words it absorbs the force generated at one end resulting
>in a lower average force being felt at the other end.

No. No 'force is absorbed'. As above.

>
>>The force applied to the anchor point lasts longer with a dynamic rope.
>
>fair enough. but it has a lower peak.

Yup, just as I said. Thank you for parroting back to me, what I said.

> If the force required to get the
>rope unstuck is more than the average then it doesn't matter how long you
>pull for. sometimes a sharp tug
>is whats required.

That doesn't make sense. You are talking about two different things. The 'average force' during a fall event only means stuff to someone undertaking a physics exam. In the real world, no one cares about 'averages'. The force applied to the anchor point, or the pinch point of a stuck rope, will change over time as a result of whats happening on the other end. The pinch point doesn't wait till you've finished tugging, work out the average force over the length of your tug, and then decide whether to come unstuck.

There will be a threshold force which must be reached to unstick a stuck rope. At the instant that force is reached the rope will come unstuck. No 'average' is required.

You are right, sometimes a rope will come unstuck with a sharper tug.

>
>>A statically applied force will result in the same resultant force at
>>the other end for a static and dynamic rope.
>
>Once the stretch is overcome in the dynamic (i.e hysteresis).

No. Not once the stretch is overcome. Any force you put on the bottom end of the rope is immediately transferred to the anchor/pinch point. Immediately. The rope stretches, but the tension is still transferred to the anchor.

>So hanging
>on the rope
>would be how you could produce this static force but this rules out the
>sharp tug which
>may be required.
>

>>Tugging on a dynamic will produce a lower peak force at the anchor point
>>than a static, though I agree with ODH that this is probably negligible.
>>
>negligible? see above.

ODH supposes that the difference between a sharp tug on a dynamic rope (~40% max elongation) and a static rope (~5% max elongation) is negligable. You suppose it is not.

To be honest, neither of you really 'know' (in the scientific sense of 'know') or have any scientifically acceptable evidence to back you up. All you have is personal experience and some self thought up theories. Basically, until someone does the experiment, neither of you can say with certainty that you are right.

However, if your supposition is as reliable as your basic physics, I'll stick to ODH's supposition.