Hello physics junkies!

I need some help calculating forces placed on my climbing gear in a slightly different than normal situation. And by slightly, I mean alot :)

I constructed a large scale rope swing under a nearby trestle. (see diagram below). Anchor is set in the middle of trestle (actually 2 anchors, one for each rope) they hang flush with the bottom of the truss system @ dead center. two ropes are attached to anchors, the other ends to your sit & chest harness. Walk back along the truss system until the ropes are nearly tight... and jump.

Now my question is this. What would the fall factor be in this type of system. I don't know how to calculate it, as you are not falling straight down onto the rope (In which case it would be 100ft fall : 100 ft rope = Factor 1) I don't think it would be as severe as a factor 1 as there is no shock loading and the process of weighting the rope is smooth and constant.

That being said, it is obviously much more force than body weight alone, as the centrifical force when you are at the bottom of the pendulum is quite significant.

All of this force is then distributed between: 2 Ropes, Two Anchors M.P., 4 anchor points (1" tubular webbing slings)

Any help??

Thanks!

Video: http://www.youtube.com/watch?v=Hp1S3g_11UA

[image]http://members.shaw.ca/emailtech/Niagara265FeetHigh.jpg[/image]

If I'm bored I might try to incorporate stretch with a Hook's law approximation.

I I do not have any pictures, but I will do my best to describe the system.

There are 4 shoulder legnth 1" tubular webbing slings tiedd with a water knot. Each of these slings are girth hitched around a different railway tie on the top of the trestle. Each sling has 1 locking biner on it.

I have 2 60' legnths of 8mm static cord. Each legnth is tied into a loop with triple fishermans. These loops are looped through 2 biners each and back down (Like a 2 piece cordalette) and tied together with a figure 8, makeing each loop 15' long before the knot is tied.

One the master point of anchor 1, 2 locking beaners are placed, gates opposed and rope one is clipped in at the half way point with a figure 8. 1" tubular webbing acts as a second sheath for this rope and extends another 12 feet incase of rubbing on the truss system.

On the master point of Ancher 2, 1 locking biner attaches a GriGri with the rope at halfway. Below the grigri, on the breakhand side is a hand ascender with a pulley on the back and has the break hand rope threaded through. This forms the 3:1 ratcheting pulley system used to haul back up. The hand ascender system sits at the top below the grigri until the swing is finished, then lowered by the weight of the hand ascender. Haul and reset 2 or 3 times until jumper is at top.

Rubbing is possible at the extreme far end of the swing right before the 2nd pendulum begins. This is why the extra sheath is added to Rope 1. An extra sheath on rope 2 is not possible due to the ratcheting pulley system. Inspection of the ropes / sheath after the first 3 jumps showed no signs of wear at all, other than a little dirt transfer from the steel.



Now, I am trying to understand dthe forces put onto the rope... If I read correctly, I have been told 2 things... the force is 3x that of gravity, or the force is 3x that of body weight? Sorry, Im not sure if I read that correctly.

The reason I ask, is I am wondering if the forces are signifigant enough to warrant me from retiring a rope used in this swing after X amount of uses, as I would with a rope that has been exposed to hard falls while I am climbing.

Thanks so much for all your help!!

Now, I am trying to understand dthe forces put onto the rope... If I read correctly, I have been told 2 things... the force is 3x that of gravity, or the force is 3x that of body weight? Sorry, Im not sure if I read that correctly.
The reason I ask, is I am wondering if the forces are signifigant enough to warrant me from retiring a rope used in this swing after X amount of uses, as I would with a rope that has been exposed to hard falls while I am climbing.

Thanks so much for all your help!!

Is there anyone with any constructive critisism of the anchor system?

I think an interesting question to ask would be what happens to that 3x force if the rope isn't completely tight when you jump. Would it go up, down, or remain unchanged?

Up.

There are 4 shoulder legnth 1" tubular webbing slings tiedd with a water knot. Each of these slings are girth hitched around a different railway tie on the top of the trestle. Each sling has 1 locking biner on it.

I highly suggest going to a wrap 3 pull 2, or wrap 4 pull 3 type system.

we don't know how creosote stained 1'' tube webbing does under load.

Good point. Normal "treated" wood literally eats metal, etc. You could probably swing on one set all day, or two days running, but if you exposed the slings to the treated wood, and then packed them away for a month (or whatever, I don't know), they might be compromised.

And you'd better tell the world's carpenters that metal nails, screws, and brackets are incompatible with threated wood. Too bad for all those decks and outdoor structures that will fall apart in a month.