UKC

Ive been told a few different ways of working this out
1) Is to Times by 10 so 1Kn = 10 kilos of force.(taught on tree climbing course)
2) Is to times by 100 so 1Kn = 100 kilos of force.(read in a thread here(http://www.ukclimbing.com/forums/t.php?n=113394) and told by somone)

I find it very hard to believe that my little wires rated to 2kn will hold 200 kilos of force, or my belay loop rated to 13kn will hold 1300 kilos wow thats more than my car weights

Very confused boy ??

Uriel:

In reply to uriel:
all wrong im afraid.

1kN = 1000N

9.81N = the weight of 1kg.

a kg is a mass, not a force.

so, the force exerted by 200kg on a wire, if just hanging there, would be 200*9.81 = just under 2000N - so a 2kN micronut could indeed hold it if the placement was secure. Obviously any sort of fall is dynamic and changes th esituation completely.

In reply to uriel:

The subject was extensively and at time heatedly discussed on here recently at:

http://www.ukclimbing.com/forums/t.php?t=105092&v=1#1441611

In reply to uriel:
Conversion chart I have gives 1Kn as something like 101Kgf, but don't forget that's kgf, not kg, so dynamic weight needs to be considered, rather than static weight (e.g. might be able to pull your car a few inches, but probably wouldn't hold it on a 6m fall!)

In reply to uriel: 1 kg is roughly 9,8 Newtons and for our purposes 1Kn can be regarded as 100kg which isn't actually that much. Fall factors are kinda cool too though. I think Petzl may have a calculator on their site.

In reply to uriel:

They are completly different units. A kN in 1000 newtons, and is a measure of force. A kilo is a measure of mass. A mass under the influence of gravity has what we term a weight. This is simply the mass of the object (in kilos) multiplied by the acceleartion due to gravity, which is around 9.81m/s^2 on the earth, and gives the force dargging the object down. You gear is rated in kN. This value is the force that must be applied for it to fail. So something rated to 2kN will hold a mass of about 200kg. However, this is only if its a static load (not moving) If you dropped a 200 kg mass your gear would be history. This is why you use stretchy ropes and so on, as they absorb some of the weight of the impact, lessening the force on the gear, and more importantly, you.

In reply to dark_star:

Indeed they are. If you take a nice big lob off a route your gear is subjected to a force of about 7kN on the top runner placement. Though it varies widely, so don't get picky.

Force is not mass or weight... but a measure of the vector quantity of something over time: or F=1kg*(m/s2).

These measurements are and indication of the point at which your harness will fail... You harness is able to apply a force equal to that which it receives until it fails ( just like the chair you are sitting on is exerting an equal force to that which your ass is applying.

Your car does not weigh 13 kN because it is not doing anything. If it were to start moving and then hit an old granny, you will have transfered a significant force to the old dear, because of the action of mass over time. In this case momentum is the force you have applied and can be measured in kN.

Got it... easy huh

In reply to finkployd: Would I break before the harnesss tho?

In reply to finkployd:

If you hang a car off a sling statically, the force applied is its weight (weight IS a force, mass is not. when you measure youre "weight", you are in fact measuring your mass, this is a common misconception). If you drop a car on to a sling then the force exerted on the sling wil indeed be governed by the momentum change and the time over which this change occurs. A car weighs the same whetehr it is moving or not.

Si

In reply to dark_star:

I think my spine would break before the 13Kn harness rating. I can't support 1.3tons at the best of times.

In reply to StonedDeranger:

energy surely?

In reply to Mat J: thanks for the replys guys!!!

OK so a 2kn wire shoud dynamically accept a force load of up to about 200kilos (or there abouts) in a factor 1 fall situation?

So in a fall situation the load should be spread over the whole system Rope, runners, gear and harness and not overly loading 1 or 2 runners.
Physics was never a strong point or maths or even having a brain...
Uriel:???/

In reply to uriel:

No! Only in a static situation, definitely not a factor 1 fall. A factor one of 200 kilos would likely break a 2kN nut.


the runners below the topt one will take little or no force but the rope, harness, and the belayer will all take some of the force off the top runner, yes.

In reply to SidH:

The force will be the same throughout the harness-rope-belayer chain.
Hoever the top runner takes almost double the force in the system. the rest of the runners have very little force applied to them.

In reply to dark_star:

Yes, but it depends on where you had tied off as too how much force would be required.

In reply to SidH:

Yes, and no... both at the same time. Weight is a measure of the force the earth (or other body) exerts as it attract you to it... mass is a measure of the amount of stuuf present in you. I meant to differentiate the use of the term 'force' when applied to a fall as opposed to mass and weight.

A car does weigh the same, but exerts different amounts of force depending on weather it is moving or not. This difference in forces dur to static and dynamic states is due to inertia, another type of force and relevent to this thread.... thanks for pointing that out.

In reply to dark_star:

Assumptions: Harness rated at 13kN
Your mass 80kg
Gravity 9.8m/s/s or 9.8N/kg

At the absolute limit of your harness you would experience an upwards force of 13000N. You would be experiencing a downwards force of 784N, and so a net upward force of 12216N.
This would accelerate you upwards at 152.7m/s/s.
That's quite fast!

In answer to the question, I think that you would break before the harness.

In reply to Mat J:

Which to put into perspective of the non-physicists here is equivalent to turning 15.5g in a jet fighter. Nice.

In reply to finkployd:


When did inertia become a 'type of force'?!
Inertia - the property of a body that resists a change in velocity (speed or direction).

In reply to Mat J:

Oh jesus... I didn't want to get in this deep... for intertia read inertial force.
i.e. the force exerted by an objects inertia on the force that is acting against it. In my car example above, the car is exerting an inertial force of linear motion upon the poor old ladies legs that are exerting thier own force against the car's bumbers trying to stop snapping her legs like thin dry twigs. She'd only popped out to get some cat food and get a scratch card.

I do appologise for not making this clear... sorry

In reply to finkployd: Look at the big picture.
(That's a pun not a dig!)

In reply to uriel:


First of all, let's get our units sorted out.

There's no such unit as a kilo. Kilo is an SI prefix indicating 1000, just as mega is the prefix for 1000000, abbreviated k and M respectively.

The SI unit of mass is the gramme, abbreviated to g.

1000g = 1kg

Force = mass * acceleration
Acceleration is the rate of change of velocity with time, metres per second squared, or m/s^2
Velocity is the rate of distance with time, metres/second, or m/s

The SI unit of force is the Newton, abbreviated to N, where
1N = 1kg * 1m/s^2

So, for a given mass, we need to know the acceleration it is subject to in order to find the force.

On the surface of the Earth, we are subject to an acceleration of 9.81m/s^2, due to gravity.

So, a person with a mass of 100kg will exert a force of

100 * 9.81 = 981N, roughly 1kN, on the floor.

To confuse matters, we say that we 'weigh' so many kilogrammes when we step on a set of scales. This is because the scales are calibrated to display our mass, assuming standard gravity of 9.81m/s^2. As it happens, gravity isn't constant, but the difference is so small as to be neglibible, especially compared to the inaccuracies of the scales.

In order to determine the load that a falling climber will present to a piece of protection, we need to calculate the acceleration they present to the gear. This is rather hard to calculate, and depends on how far the climber has fallen, how much rope he falls on, how far he is above the gear, how dynamic the rope is, how much friction there is in the system.

It should be pointed out that conversion from fall factor to acceleration is not simple. Certainly, you can't say that a factor 1 fall will simply cause an acceleration of 10m/s^2, as you suggest with your 200kg and 2kN figures.

A 2kN rated piece of gear should just hold 200kg UNDER GRAVITY ALONE (F = ma, so F = 200 * 9.81 ~ 2000). A factor 1 fall will present an acceleration much greater than this. For a more sensibly sized climber, say 75kg, it will hold a fall presenting an acceleration of 27m/s^2, or 2.7 times gravity.

That's why krabs and slings are rated around 25kN, so that they will survive a 100kg climber applying an acceleration of 250 m/s^2, or 25 times gravity. And, since the climber also experiences this acceleration (since action and reaction are equal) this is very likely to break bits of you.

If all this is baffling, just remember that you really don't want to fall any distance on anything rated at less than about 5kN. At least I wouldn't want to. And the higher rated it is, the better.

And then there's placement...

In reply to captain paranoia:

Isn't it safer to just, not fall off?

In reply to captain paranoia:


The SI unit of mass is the kilogram, abbreviated to kg.

http://physics.nist.gov/cuu/Units/units.html

The CGS (centimetre-gram-second) unit of mass is the gram (or gramme).

Nick

In reply to Mat J:


Pedant.. But yes, get slopy with my use of words. Was typing fast while hungover.

In reply to SidH:

I'm feeling pedantic now. When you measure your weight you are not measuring your mass, you are measuring your weight, or to be more correct, the force your feet are exerting downwards on the scale. This is translated into mass on the assumption that g=9.81m/s^2 or whatever. If you bounce on the scales the reading changes as the force changes. Your mass does not, nor does your 'weight' really which is a force soley due to gravity.

In reply to captain paranoia: You've got force and acceleration the wrong way round in most of your explanation. You can't have an acceleration without there being a force applied, but applying a force doesn't necessarily give rise to an acceleration. I am applying a force of roughly 780N to the chair I'm sitting on, but luckily the floor is pushing back with the same force so the chair isn't going anywhere. Which is handy, otherwise I might find sitting on it rather tricky. You'd better hope that a similar equilibrium situation applies to any gear you fall on to, otherwise it's going to be following you down the mountain!

You've also forgotten that the top piece of gear experiences slightly less than double the peak force experienced by the climber, due to the pulley effect (with some mitigation from friction in the system). Thus if you take a major whipper on to a 25kN rated piece of gear and it just holds, then you will probably experience a peak force in the region of 14kN. I seem to recall reading that this is around the limit of what the human frame can sustain without significant risk of serious injury.

In reply to Nick Lambert & Martin W:

SI unit of mass is the kilogramme. Agreed. I'm trying to think of another unit where the 'base quantity' requires a multiplier prefix. I think kg may be the only one...


Acceleration/force. Agreed. I started my explanation from the point of view of an arresting climber, and the accelerative forces on them. Unfortunately, that dragged the rest of my wording along those lines. I was also rather hurried, as about to get thrown out of work.

And you're right, in my haste I had (again) neglected the pulley effect.

In reply to captain paranoia:

Actually, I went home thinking 'well, that was a crap explanation, there'll be loads of comments tomorrow morning'. Only there weren't...

In reply to uriel: Oh my God its true what they say about people on the internet.....there have been some proper sad act responses to the question I'm afraid! why do people have to try and look clever - (reminds me of some uni lecturers.) I mean: 'the inertia of a moving car' - what the fcuk?!! the guy asked a relatively simple question why try and blagg him. It's true falls are quite hard to analyse; lots of variables. But the simple answer is:

1KN = 1000 N = 100Kg(roughly- forget about exact value of g)

So yeah you were pretty much right, something rated at 22KN like a crab say, would *statically hold* your car easily...BUT obviously if you drop your car a few metres onto the crab it could well fail.

From here the rest is intuitive really.

In reply to Steve Ramsden:

Thank you for a sane and nonargumentative response thats easily understood, I wasnt looking for a lecture or an argument, just one decent reply so ta

Uriel
The unconfused one (now)

In reply to Steve Ramsden:


Only if the only force acting is that of gravity.

What I tried to explain (and I guess your 'sad act' comment applies to me), is that the dynamic behaviour of falling generates forces much higher than that of gravity. And these forces ARE hard to calculate.

The OP also demonstrated a misunderstanding of how fall factor relates to the peak loading of protection, and it was this that prompted me to try to explain, in a helpful manner (although I failed), some of the mechanics behind it.

In the context of load ratings of climbing protection, the dynamic element of the load is far more important than the static force of gravity.


I'd disagree. Very non-intuitive things happen with pulleys. For instance, if someone lowers me from a piece of gear, the gear experiences a force of twice my bodyweight. However, if I then grab the up rope, and lower myself, the load on the gear is only my bodyweight. I'd suggest this isn't intuitive. You have to think about it a bit.

If I misunderstood the original post, I'm sorry. I assumed that, since this is a climbing site, the OP wanted to understand how protection load ratings could be related to a climber's bodyweight, in a climbing (falling...) situation.

In reply to captain paranoia:

Don't get too paranoid about it Cap'n

In reply to all:

Although extremely interesting, this thread makes my head hurt. It has, however, put me in the mood to go and get very, very pissed.
Thanks

In reply to Murdy:

Well, I'm glad to have been of service to someone...

In reply to Steve Ramsden:
Sorry mate... didn't mean to offend you.

Just wanted to explain what the difference between kilos and kN were... the car thing was just an attempt to inject some humour.

Anyway, I was a student at Hull yonks ago... great place, until I went back and thought, jeeesus... I spent three years of my life here.

Nice pubs though, especially in the old town. Lots of very fond memories... you'd better enjoy student life, everything sucks hard after that.