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Chockstone Forum - For Sale

Buy and Sell Used Climbing Gear Please do not post retail SPAM.

 Page 3 of 5. Messages 1 to 20 | 21 to 40 | 41 to 60 | 61 to 80 | 81 to 92
Author
Used ropes for sale?

gonzo
30/04/2007
6:58:32 PM
anthony, i think that you should reconsider your theory (and possibly stop listening to the guy at st. p's).

now i haven't done physics for a while, but as much as i remember the force should be evenly distributed along the length of the rope no matter where it decides to stretch and where it doesnt. lets say 1m of the rope doesnt have any stretch left in it, you still have 49m of stetch (50m rope) to absorb the force - the force doesnt all get bunched up in one spot.
kieranl
30/04/2007
8:41:08 PM
The real problem with doing long jumps is that you give more space for chaos to change things. We like to think that things are predictable and if we are meticulous in our design then we can predict the outcome. This is complete garbage, especially when you give enough room for different waveforms to interact.
Do I know what I am talking about? No. But that doesn't make it wrong. I hope you don't do it but I hope that you get away with it if you do. I wouldn't do it in a pink fit.
dalai
1/05/2007
9:24:26 AM
On 30/04/2007 mousey wrote:
>On 30/04/2007 rightarmbad wrote:
>>How do you get a fall factor greater than 1? You are falling the same
>distance
>>as the length of rope out. Fall factor 1.
>>What sort of forces have you figured for your anchor rope? They gotta
>>be pretty high.
>
>you are making the very incorrect assumption that the launch point is
>at the same height as the anchor.
>
>
>
>>one thing you need to remember is that....
>
>holy shit you're right! i hadnt thought of that!
>come on seriuosly guys....
>
>what you should do is start your own thread where you can pretend to have
>any idea about how im going to rig this, then everyone can throw in their
>2c about how thats a bad way to do it.

Hmmm...

You post a topic about a rope jump on a public forum in a National Park.
You elicit responses via some information and enjoy the discussion.
You get responses you don't agree with based on others speculating as to what you plan on setting up.
You throw a tantrum telling people off for discussing the jump with you...

Sabu
1/05/2007
9:32:50 AM
On 30/04/2007 mousey wrote:
>>>All very interesting, but
>>>when will we see the photos posted?
>
>how does never sound?

you lie, if you get away with it you'll be so proud you won't be able to resist sharing the moment with us.. ;)
TonyB
1/05/2007
9:50:23 AM
>
>you are making the very incorrect assumption that the launch point is
>at the same height as the anchor.
>

Why so cagey about telling us exactly what you plan to do ? Do you want advice and comments from the forum ?

westie
1/05/2007
10:01:31 AM
mousey ... mousey ... mousey ... mousey... MOUSEY MOUSY MOUSEY MOUSEY MOUSEY... jump, jump, jump, jump, jump, jump, JUMP JUMP JUMP JUMP JUMP JUMP! JUMP! JUMP! JUMP! ahhhh.just like being a kid again. c'mon mousey come up with the goods and shut us all up.

PreferKnitting
1/05/2007
10:05:01 AM
On 1/05/2007 Sabu wrote:
>you lie, if you get away with it you'll be so proud you won't be able
>to resist sharing the moment with us.. ;)

By 'getting away with it...' Do you mean if he doesn't get caught by authorities or surviving the jump?

Either way, mousey, be careful. We like your ramblings and photos and sh*t. Maybe a crux article about the experience would be worthwhile.
TonyB
1/05/2007
10:17:58 AM
> ..all
>your assumptions about stretch and load that a rope can take are based
>on assumptions that the forces will get distributed evenly throughout the
>rope.

Ignoring the weight of the rope and transient wave effects, the force IS the same throughout the rope. What may vary is stress (force/unit area), if the cross sectional area varies at a point; and strain (deformation), if the Young's modulus varies due to weakness at a particular point. There is no "concentration of energy at one point". If there is sufficient necking or a sufficiently weak spot in the rope, the rope will break if the yield stress is exceeded. This is no different to a very short fall with a similar fall factor.

anthonyk
1/05/2007
1:47:19 PM
On 1/05/2007 TonyB wrote:
>> ..all
>>your assumptions about stretch and load that a rope can take are based
>>on assumptions that the forces will get distributed evenly throughout
>the
>>rope.
>
>Ignoring the weight of the rope and transient wave effects, the force
>IS the same throughout the rope. What may vary is stress (force/unit area),
>if the cross sectional area varies at a point; and strain (deformation),
>if the Young's modulus varies due to weakness at a particular point. There
>is no "concentration of energy at one point". If there is sufficient necking
>or a sufficiently weak spot in the rope, the rope will break if the yield
>stress is exceeded. This is no different to a very short fall with a similar
>fall factor.

force at each point was what i was talking about, which isn't the same througout the rope. and yes there is concentration of energy at a point if its properties are different, either due to a change in local force or if its stretching properties are different. if it gets taken up over a shorter time the force is higher. try soaking a small section of a rope in hard glue and seeing how long you last falling on it.

but maybe its not as bad as i thought. the other point i was getting at is there's a whole lot more energy around, aside from air friction you've got 25x more energy to absorb than in a 10m fall (KE=mgh). and if the stretching properties of your rope are different to what you assume you can have parts of the rope take up more energy with time (force) than you assume. and its a runaway process, if strands get damaged the load on the remaining strands at that point gets increased and could cause the rest to fail.

Sabu
1/05/2007
2:05:56 PM
On 1/05/2007 PreferKnitting wrote:
>On 1/05/2007 Sabu wrote:
>>you lie, if you get away with it you'll be so proud you won't be able
>>to resist sharing the moment with us.. ;)
>
>By 'getting away with it...' Do you mean if he doesn't get caught by authorities
>or surviving the jump?
haha well i guess both apply don't they!

>Either way, mousey, be careful. We like your ramblings and photos and
>sh*t. Maybe a crux article about the experience would be worthwhile.
ja i second that!


On 1/05/2007 TonyB wrote:
>Why so cagey about telling us exactly what you plan to do ? Do you want
>advice and comments from the forum ?

conspiricy theory >> ya know wat i reckon is that mousy got this idea, got very excited about it, began planning it and posted this thread in his excitement (obviously).
now with a few of the conflicting posts up he's beginning to consider the other aspects to this (such as the parks stuff), and perhaps, as a result, starting to cover his tracks in preperation for his "get away"! lol am i close mousy? the less people know the better?! lol.
TonyB
1/05/2007
4:44:54 PM

>force at each point was what i was talking about, which isn't the same
>througout the rope.

The simplest demonstration that force is uniform throughout the rope, is to tie spring balances between lengths of rope, and pull ... all balances will show the same force. Stress in the rope is force divided by cross section area ... this may change if the cross sectional area is not constant.

> if it gets taken up over
>a shorter time the force is higher.
> ... there's a whole lot more energy around

The kinetic energy of the falling body/person is equal to it's (mousey's) initial potential energy, which is proportional to the height he falls (ignoring energy lost in heating the air.) The force in the rope is proportional to the distance the rope stretches, not the time in which it does so. You may recall: Energy = Work = Fs.
We assume the height of the jump is equal to the length of the rope. Since the stretch length is proportional to the length of the rope, the force will be the same for different height jumps, ignoring air resistance. Air resistance will reduce the force for bigger jumps.
Always make sure there is no damage to ropes, no matter what height you are jumping or falling from.


anthonyk
1/05/2007
7:04:35 PM
On 1/05/2007 TonyB wrote:
>(ignoring energy lost in heating the air.) The force in the rope is proportional
>to the distance the rope stretches, not the time in which it does so.
>You may recall: Energy = Work = Fs.
>We assume the height of the jump is equal to the length of the rope. Since
>the stretch length is proportional to the length of the rope, the force
>will be the same for different height jumps, ignoring air resistance.

you've painted yourself into a corner there. if the rope always stretches the same amount, according to your assumptions its always absorbing the same amount of energy. but the energy changes with the height the person is falling. the amount the rope stretches is based on the energy it is absorbing, but its complicated. note a person weighing 0kg falling 0m causes the rope to stretch 0cm, regardless of how long the rope is..

also i think you are also making assumptions about a static system that don't hold when its moving, eg the tension is not necessarily equal throughout the rope when its stretching.

wallwombat
1/05/2007
9:05:16 PM
The thing that cracks me up is that while everyone on this thread is debating over the rope jump, two people are having a really intense arguement about the physics involved.

As John Lennon said ' What ever gets you through the night......'


Super Saiyan
1/05/2007
9:41:19 PM
You can argue physics til the cows come home. I dont care how stuff works, just that it does. Big kudos for having the balls man!

Sabu
1/05/2007
10:15:08 PM
On 1/05/2007 wallwombat wrote:
>The thing that cracks me up is that while everyone on this thread is debating
>over the rope jump, two people are having a really intense arguement about
>the physics involved.
yea thats pretty funny!

Andrew_M
2/05/2007
12:42:37 AM
On 1/05/2007 wallwombat wrote:
>The thing that cracks me up is that while everyone on this thread is debating
>over the rope jump, two people are having a really intense arguement about
>the physics involved.

Forget arguing the theory...there is only one real way to get these two from each others throats...mousey has to try all the different combinations and when the rope snaps we'll know it shouldna been done that way. Are ya up for it mousey or are ya a big wuss?
kp
2/05/2007
9:29:08 AM
this is a great thread.

It deserves to be bumped.
TonyB
2/05/2007
11:04:51 AM

>you've painted yourself into a corner there. if the rope always stretches
>the same amount, according to your assumptions its always absorbing the
>same amount of energy.

Please read my post again "stretch length is proportional to the length of the rope". That is, a rope twice as long stretches twice as much. The calculator on this page may help you get a better appreciation: http://www.getbeta.com/fall_factor.asp

As people correctly point out, my discussion is just theory. I wouldn't trust it without a real world experiment. 75Kg dummy with load cell, accelerometer and transmitter should do the trick ... and for not much more than the cost of 250m of rope. Alternatively, it might make a nice project for a mechanical engineering student with a uni lending the hardware. A plot a maximum load vs fall height would be most interesting ... it's a wonder it hasn't already been done.

IdratherbeclimbingM9
2/05/2007
11:09:15 AM
>it's a wonder it hasn't already been done.
It has but their tiny ropes had a nylon canopy attached.
Steve M
2/05/2007
12:14:36 PM
Talk it up mousey.

It will never happen.

 Page 3 of 5. Messages 1 to 20 | 21 to 40 | 41 to 60 | 61 to 80 | 81 to 92
There are 92 messages in this topic.

 

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