# Life-saving 'Pole' for Cliff-edge drop

• Twinbee

#### Twinbee

My own weird idea I thought of about a year ago. Be interested to see the answers...

A longish strong pole (about 2 metres) could theoretically save a life if someone used it when jumping from a cliff edge. It would be used just before they hit the ground to decelerate themseves more gently (think of them pointing it downwards, and sliding down the pole near the ground). Also assume a flat surface on the ground and that the person falling is from a steep drop (terminal velocity).

How long would this pole need to be to prevent death? Would a strong carbon-fibre material be suitable for the pole?

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The length of the pole depends entirely on the friction between it and your hands (I am assuming you are holding the pole?). You can make the friction larger by squeezing harder, but I can only imagine one thing happening: you squeeze so hard that your hands become too hot with friction. You let go. Splash. The end.

Assume friction isn't a problem. Perhaps you can have gloves on, and the material of the pole can be made to allow maximum friction possible.

So how are we supposed to know what the maximum friction possible is? In theory it would be infinite, and you would stop instantly. That is of course not a physical reality.

Nevermind what the maximum possible friction actually is. I'm just speaking theoretically.

Obviously, one would use a finite amount of friction so as not to stop instantly (and because it's impossible anyway to stop instantly) like you say.

Again, assume friction isn't a problem.

It's exactly the principle used in our safety harnesses. The cable from your harness to the anchor point is sewn in loops, when you fall the stitching breaks to gradually extend the cable and slow you down.

Assuming you have fallen a long way and have reached terminal velocity (for a person about 200km/h) then all you have to decide is how much g force you are prepared to use to slow them.
Then it's just the distance it takes someone going at this speed to stop given that accelaration - which is just the same question as how high will a cannon ball fired upwards go. Use v^2 = u^2 + 2as

For a 200km/h skydiver slowing at one 'g' it's about 150m!
You could survivably handle much higher 'g' loads for a short time.

Won't a safety net do a better job?

How do you make sure the person won't land in the middle of the pole? Imagine a man landing on the pole, with legs on both sides...

I am assuming this is just a hypothetical question; even if you would succeed in calculating the required pole length, which material, etc, I still wouldn't recommend you trying it out :p
(Furthermore, who happens to carry around a 150m long pole in their back pocket, just in case you happen to fall of a cliff?)

Hi Twinbee!
A longish strong pole …

Well, I'd go for a weak pole …

you want the pole to absorb as much energy as possible.

In addition, I think it should be filled with something like fudge, which will spill out and not only help cushion your landing, but provide you with food to keep you alive until rescue comes

In addition, I think it should be filled with something like fudge, which will spill out and not only help cushion your landing, but provide you with food to keep you alive until rescue comes

:rofl: ROFL! :rofl:

It's exactly the principle used in our safety harnesses.
Aha interesting.

For a 200km/h skydiver slowing at one 'g' it's about 150m!
You could survivably handle much higher 'g' loads for a short time.
Okay we're getting closer. What's v, u, a, s in your equation? Here's a handy quote which fills in the gaps:

Take from here
Humans can survive up to about 20 to 35 "g" instantaneously (for a very short period of time). Any exposure to around 100 "g" or more, even if momentary, is likely to be lethal, although the record is 179.8 "g".

Do we just divide 150 by 35? (which would make a 4 metre pole).

Won't a safety net do a better job?
Yes.

How do you make sure the person won't land in the middle of the pole?
The person who's skydiving would carry the pole with them as they are skydiving. They would hold onto it perhaps half a metre from the top to prepare for the deceleration phase.

I am assuming this is just a hypothetical question; even if you would succeed in calculating the required pole length, which material, etc, I still wouldn't recommend you trying it out :p
Lol, I bet it will give someone ideas, though I am not looking to take up a career as a stuntman any time soon.

(Furthermore, who happens to carry around a 150m long pole in their back pocket, just in case you happen to fall of a cliff?)
That's only for 1 G's worth. Humans can survive up to 35, and even up to 180 G as said earlier.

Well, I'd go for a weak pole ...

you want the pole to absorb as much energy as possible.
This is an effective but different approach. For one thing, the pole can't then be reused. The way I'm thinking of, one would slide down the pole just after it hits the ground.

In addition, I think it should be filled with something like fudge
Agreed.

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V2 = u2 + 2as
is a well known equation, V is the final velocity, u is the initial veleocity, a is accelration and s is distance (the Latin and German words for space/distance begin with s)

So since final velocity is 0 (ie you are stopped) and accelration is negative (slowing down) you have
u2 = 2as or distance = sqrt(u2/2a)

200km/h is about 55m/s, assuming (de)accelearation is one g (9.8m/s2)
s = sqrt( 55/2*9.8) = 150m

Of course you can use a much shorter pole and a higher deceleration - if you do remember to take into account the person's height, remember their feet hit the ground before their hands.

This is also important with safety lines that attach between your shoulders!
The rules here are that you must wear a safety harness if you are more than 3m/10ft up.
The rules also say that the line must stop you within 1.5m (5ft) and must be attached between your shoulders.
So add in an extra foot for the hooks etc and if you are any taller than 6ft you are are going to hit the ground anyway!

There was a report of one guy who died after a fall of only 20ft, he had used 3 life lines for extra safety - but had put them end-end!

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Hi Twinbee!

Well, I'd go for a weak pole …

you want the pole to absorb as much energy as possible.

In addition, I think it should be filled with something like fudge, which will spill out and not only help cushion your landing, but provide you with food to keep you alive until rescue comes

The fudge would simply provide treats to the paramedics that had to sponge up the splattered remains caused by failure of their "Acme Pole Decelerator" device. (Guaranteed, or your money back.)

Er uh, they make a fairly reliable device just for this purpose; it's called a “parachute”.

Cheers all,

Another thing to bear in mind is that the velocity doesn't have to be exactly zero when they hit the ground, as their legs can provide additional deceleration. I've therefore added just an extra 1 metre to the result in each case. One may wish to add an extra 0.5 or even 1 metre just in case.

However, one danger not yet mentioned is that you can't fall sideways, as the bottom of the pole has to hit perpendicular to the ground. I'm not sure whether a perfect angle can be always achieved every time.

Anyway, all things considered, I work it out to to a curious but slightly 'disappointing' 5.4 metre pole then (assuming 35 G is the maximum for a great chance of survival). However, this drops to about 3 metres if you want an estimated 50% chance of survival, and for the guy who survived a 180G world record, a 1.86 metre pole would be sufficient!

Here's the math I used (thanks to mgb_phys):

V^2 = u^2 + 2as
s = (v^2/2 - u^2/2) / a
RequiredPoleHeight = (v^2/2 - u^2/2) / a + 1

RPH = (0^2/2 - 55^2/2) / (-9.8*35) + 1
RPH = (0^2/2 - 55^2/2) / (-9.8*80) + 1
RPH = (0^2/2 - 55^2/2) / (-9.8*180) + 1

Er uh, they make a fairly reliable device just for this purpose; it's called a “parachute”.
Oh I can think of numerous advantages with the pole approach. For one, it's less expensive. Two, parachutes waste time since the pole allow maximum speed towards the ground and minimizes the usually tedious deceleration stage to the last few milliseconds. Also, have you ever seen parachutes filled with fudge? It would probably cause the parachute not to open in the first place!

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You should be able to make a reasonable approximation by simply jumping off and having a rope playing through your hands. At the point you would use the pole, clench the rope.

See if you can win an egg-drop competition using a folding pole (with spring loaded elastic articulation, and fins or tail to orient it).

It reminds me of starship troopers drops, avoiding decelerating before the last moment. I wonder how realistic the necessary friction control, pointing precision and material stress are. Hmm.. if you know the soil conditions, you mightn't need the sliding grip.

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I'm truly not being sarcastic here, but i think one would be better-off having some type of gas-inflatable something to help absorb impact between oneself and death below.

I'm truly not being sarcastic here, but i think one would be better-off having some type of gas-inflatable something to help absorb impact between oneself and death below.
Oh like a bouncy castle? ;)

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Ouch.

Of course the most likely out come of such a device, would be impalement on a splintered pole.