# Can you help me with some portable physics?

Asked by Nullo (21944) October 15th, 2012

I’m working on a prototype deer stand safety harness that would use a rope and some counterweights to lower the person in question from his tree stand to the ground. I need a formula to calculate how much weight I’m going to need to offset a falling hunter while avoiding any sudden stops.

The plan is to secure the harness to a climbing rope (1200lbs test/120lbs working load, which seems like it ought to be enough), pass the rope through a carabiner strapped to a tree trunk, to a set of weights probably kettle bells in the prototype on the ground.

Don’t look at me like that. I took chem and bio in high school, not physics.

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Fascinating ! I don’t have a specific answer for you but I bet that sort of problem has been solved well by rock-climbers. I think the solution is to use climbers’ rope that absorbs the shock of decelerating a falling person. The rope they use has to be changed after a fall as the inner portions will have stretched to soak up the energy.
Unless your kettle bells kick in immediately the sudden lurch of running out of slack rope could injure the faller.

dabbler (16570)

This is for homework, isn’t it! Shame on you!

I kid, i kid. If i recall correctly, if you were to use a weight equal to the person, you’d get a system that was essentially ‘weightless’, as they could move up or down easily and stay in position. However, as this would be not very useful for a lowering mechanism, i think something just a little lower should work okay. So if they’re, say, a 200 lb person, maybe 170–180 lbs. This is, however, a very rough estimate, and would need to take into account the resistance of the carabiner, but it’s a possible place to start.

And while it is close to the purview of rock climbers, usually the ‘counterweight’ is a person and thus can let rope out as needed (as opposed to flying upward as the other descends), or the system uses a stationary rope which the climber can let out themselves to go down, and so not actually that useful for this case. Though it’s been a while since I’ve climbed, so I could be a little off.

BhacSsylan (9522)

With pulleys, you can greatly reduce the counterweight need.

It’s the rope equivalent of gears on a bike or moving a rock with a lever, where you trade distance moved for power.

How a Block and Tackle Works

Also I would look at rock-climbing resources, I bet they solved this long ago.

jaytkay (25767)

My dad knows tons about rock-climbing, I might be able to ask him. I’ll try and tell you what he will probably say.

There probably is no amount of weight that can guarantee a slow descent. Either the weight is enough to completely stop you from falling, or it’s not enough to stop you from falling.

A counterbalance with the same mass as you will theoretically be able to balance, but once you are falling, your own momentum is enough to continue raising the counterweight and allow you to fall further.

Since you will probably prefer no descent over a fast descent (aka fall), you might as well skip the weight and directly attach the rope to a fixed object.

He would definitely recommend dynamic ropes. A dynamic rope is a rope that is able to stretch, thus making the abrupt stop less abrupt. Just make sure that the dynamic rope is not so stretchy that you hit the ground before it finishes stretching.

Anyway, even though I don’t know the specifics, you can count on the fact that rock-climbers have figured out all of this stuff.

PhiNotPi (12670)

I think you are trying to re-invent one of these:
http://www.tasco-safety.com/fall-protection/retractable-lanyards.html

RocketGuy (10836)

Thanks all! I’ll be following these leads.

@PhiNotPi @RocketGuy The problem that I’m trying to avoid is time in the harness. The standard deer stand harness is not a very good one: two of the straps are placed in such a fashion as to severely restrict blood flow when stressed, leaving mere minutes of consciousness in which to clamber back onto the stand, or find and use an emergency whistle. Most hunters are in those deer stands by themselves, sometimes miles from help, and unconsciousness can become a permanent state.

Nullo (21944)

@RocketGuy that looks like just the thing, unimposing when not needed, catches like a seat- belt when needed.

dabbler (16570)

or