If you jump off a bridge 50m above the water does the water need to be 50m deep?

[JimmyJockstrap]

how deep would it have to be proportionally to how far you fell?

 

[HallsofIvy]

There are a number of things missing here! You are, I think, saying that you don't want to hit the bottom.

You can pretty much ignore air resistance and calculate your speed when you hit the water but after that things get complicated. You MUST take into account the resistance of water since that is what will slow you. That depends in a complicated way on the cross section: how fat you are perhaps as well as the angle at which you hold your body. I think "surface effects"- exactly how you hit the surface of the water- would also be important. If you landed "flat" (what kids call a "belly flop") you might not go at all deep.

 

[arildno]

Also, it'll hurt you to collide with the water from that height..

That is a far more relevant concern than the risk to hit some sharp rock lurking at the bottom or be bitten by a grumpy flounder..

 

[rewebster]

Also, it'll hurt you to collide with the water from that height..

That is a far more relevant concern than the risk to hit some sharp rock lurking at the bottom or be bitten by a grumpy flounder..

yeah,...I think that I'd let all my friends jump off the bridge first...

 

[arildno]

The water needs to fed first to become compliant and easy-going towards you..

 

[rewebster]

are you saying even water has to be "PC" anymore?------what's this water coming to?

 

[rbj]

you can do maybe 15 meters as less than an expert diver (something I've done and it's scary enough), but 50 m is 164 feet and I've read *many* stories of people jumping off of bridges from 100 to 200 feet, thinking that they could just swim away and their bodies washed ashore instead. you have to hit the water perfectly straight, like a pencil. even though impact velocity increases with the square root of height, it's really not velocity that should scare you but kinetic energy you have when you splash, which is linear with height.

 

[Math Jeans]

It also takes into account whether you have a full tank of air. The upward force that the water exerts on you is equal to the weight of the displaced water. You must also take into account the resistance of the water. I would say that generally given the material of the human body, you would not make it down into the water 50m after jumping 50m, and there is no direct multiplier to find how deep you go.

 

[arildno]

Of course, if you could jump with an equipment that yields a negative splashification factor to the impacting water (i.e, accelerating the water downwards as you meet it), you could do well.

I saw such an equipment last time I visited Marks&Spencer.

 

[rewebster]

in other words, don't jump with wearing a parka or a snowsuit

 

[Math Jeans]

I think you guys are missing the point. He isn't asking how to survive the jump, he's asking how far down you would go. Just a question, are you looking for general equations for the water's effect on a human (which would end up being really complicated), or explanations like what we've been giving you?

 

[rewebster]

(I know)-


-Halls and rbj gave pretty good ones with the information given


_______________________________

What was that contest where all those people dressed up and jumped off of that dock? ---some commercial for??

 

[JimmyJockstrap]

mean hypothetically there is this bridge. Say someone tries to murder or maim me when I am on the middle of the bridge, nowhere to run. What is the safer option. cop a beating or jump 13.3m?

It is 13.3m above the water. I know someone who jumped and survived off it.

 

[rcgldr]

Olympic diving pools which include a 10 meter platform require water depth to be at least 16 1/2 feet, about 5.03 meters. The Bellagio "O" pool is 25 foot deep for dives from a 60 foot platform. Cliff divers sometimes dive into shallower water, but curve underwater to avoid hitting bottom.

 

[Math Jeans]

(I know)-


-Halls and rbj gave pretty good ones with the information given


_______________________________

What was that contest where all those people dressed up and jumped off of that dock? ---some commercial for??

Lol. Is that the strategy? Answer the question and then use the thread space to talk about whatever you want related to the subject?

Sounds fun . I actually know a diver myself. Never seen her dive, but sounds cool.

 

[rbj]

mean hypothetically there is this bridge. Say someone tries to murder or maim me when I am on the middle of the bridge, nowhere to run. What is the safer option. cop a beating or jump 13.3m?

It is 13.3m above the water. I know someone who jumped and survived off it.

13.3 m is 43 ft which is not too big of a deal. (50 m is a big deal, enough to make me incontinent.) if the water is 4 m deep or more, it's unlikely you'll touch the bottom with your feet.

jumping off a cliff (or bridge) at approx. 13 meters is not too bad (maybe a little scary) for someone in good health, with good coordination, and can swim well. I'm 51, in okay shape, but no athelete. stick your arms out at first so you can use them to help you stay vertical on the way down. at the last split-second, put your two feet together and arms up. i often forget to put my arms up and i get this "slappy" rash that goes away in a day. but it stings.

besides the depth of the water, the quality of the water is a concern for me. that's why i don't do bridges because they are generally over rivers which are not that clean. in Burlington Vermont, at Lake Champlain, were i live, there is a bike bridge over the mouth of the Winooski river that a bunch of adolescent kids like to jump off in the summer, but the water there is, in my opinion, disgusting. i keep telling these kids to jump off the cliffs at Lone Rock Point or at Red Rocks Park. clean water and an adrenaline kick for the adrenaline junkie.

so, if i was facing a likely stabbing death on a bridge i estimated to be about 13 meters up, i might jump. but the hypothetical is hard to suspend disbelief.

 

[Loren Booda]

Acapulco cliff divers jump from 59 feet into (if they are lucky) 9 foot swells, maxing out at 80 mph. Immediately after entering the water one must spread out, maximizing one's bodily surface area.

Jumpers from hundreds of feet must do so feet first and protect their genitals and face. My guess is that the impact is proportional to the speed of entry, and that speed squared is proportional to the height jumped.

 

[rcgldr]

Acapulco cliff divers jump from 59 feet into (if they are lucky) 9 foot swells, maxing out at 80 mph. Immediately after entering the water one must spread out, maximizing one's bodily surface area.

As I previously mentioned, the divers just curve their bodies so that they turn underwater to avoid hitting the bottom. The idea is to reduce g forces, not increase them.

 

[Loren Booda]

Thank you, so noted.

 

[rbj]

Acapulco cliff divers jump from 59 feet into (if they are lucky) 9 foot swells, maxing out at 80 mph.

better check your numbers, Loren. i don't know how you can accelerate to 80 mph (from 0 mph) in 59 feet on planet Earth.

30 ft -> nearly exactly 30 mph and then quadruple the height for each doubling of speed.

My guess is that the impact is proportional to the speed of entry, and that speed squared is proportional to the height jumped.

depends on what you mean by "impact", but i believe the energy in the splash that can potentially break a bone (or worse) is proportional to the square of the speed of entry (which is proportional to the height). i.e. twice as high is twice as "bad", not $\sqrt{2}$ times as "bad". 80 mph is 4 times worse than 40 mph.

 

[tony873004]

HallsOfIvy already mentioned surface effects. I wonder what the difference would be between choppy water or "glass", or between fresh or salt water. I believe the belly-flop results because of the surface tension of the water, and I believe choppy water has a lower surface tension, hence less of a flop causing you to go deeper. But I'd prefer deeper to a painful flop, which can kill you. I've heard that when people jump from the Golden Gate Bridge, it's as if they hit concrete. I've also heard that the few survivors all land feet first, at an angle almost vertical, which I guess would make them go rather deep. That would not be a fun swim to the surface :(

 

[rbj]

I believe the belly-flop results because of the surface tension of the water,

i don't see what the surface tension of the water has to do with the orientation of the diver a millisecond before touching the water. the belly-flop happens because, for whatever reason, the diver or jumper was unable to get into a sufficiently vertical orientation to begin with or to maintain the vertical orientation on the way down. all you have is the air to grab on to, and if you don't know how to use parts of your body (like your arms and hands) as airfoils and to control those surfaces in such a way to maintain vertical orientation, you might hit the water in any orientation. except for one orientation (vertical, feet first) with very little tolerance for error, all other orientations may very well kill you hitting the water at 80 mph (which is what you get for 220 ft). if you like doing the splits, orient one leg slightly in front and the other slightly behind and remember to say "ouch" as your femurs are dislocated from your hip.

I've heard that when people jump from the Golden Gate Bridge, it's as if they hit concrete.

i've heard that too, but liquid H₂O is always softer than concrete. still might not be soft enough.

I've also heard that the few survivors all land feet first, at an angle almost vertical,

i would affirm that. if they are not vertical, there are side "sheering" forces that will bend your body, or parts thereof, rapidly and potentially in a direction that you won't like.

it's funny (or at least, ironic) that most of the few survivors were suicide attempts and nearly every person who thought they could jump it and be okay was killed.

which I guess would make them go rather deep. That would not be a fun swim to the surface :(

hold your breath. i'll bet they go no deeper than 25 ft. could be wrong.

 

[rewebster]

I dove off of about a 40-45 foot cliff (once)------went in double tight fisted, and one shoulder was sore for about a week when it pulled out and then back behind me. I didn't haven't the strength to hold it in front all the way through.

 

[rbj]

you have more testosterone than me. I've done that height about 8 different times, but always feet first. other than slappy rash (because i didn't lift my arms up at the last quarter second), i hadn't suffered anything. oh, on second thought, i cut my toe (very cleanly) on a sharp silicate poking out of the rock cliff where i launched from. at that location, i now wear sneakers when i jump. at (South) Burlington's Red Rocks park, they also have nice cliffs at 60 and 76 feet, but I'm too much of a pu_ssy to jump those.

 

[rewebster]

I did jump twice just to test the water and to see what my feet would do. No one else did a dive though. ah---the idiocy of youth--the good old days!

 

[rcgldr]

Aerodynamics at the speed that high divers achieve doesn't offer enough resistance to allow a diver to remain vertical control. Almost all high divers going in feet first use one or two controlled front flips with a 1/2 twist at the end, and vary the amount of tuck at the end to end up vertical. The diver can spot the water the entire time with a front flip with 1/2 twist, and if doing a back flip for the last 3/4 of a flip if doing a full twist. Trampolinist rely on the same method of landing facing downwards (back flip) to spot the trampoline.

Head first dives are usually swan dives where the diver can spot the water the entire time, or 1 1/2 front flips, where the diver has to look downwards for the last half flip to control the landing.

update - Although sprinkler systems are used to help olympic type divers spot the surface of the pool, some of their more difficult manuevers make spotting and adjusting difficult, which is why you see occasional mishaps. In comparason, the high divers that jump from 60 feet or more are more interested in entry position rather than complexity of a dive for a score, and mishaps are much rarer.

The hands are placed one behind the other with the inner hand wrapping the fingers around the outide of the palm of the outer hand, and the outer hand fingers wrapped around the fingers of the inner fingers. Many divers use a wrist brace. The purpose of this method is to reduce the splash, but puts more stress on the wrists. A cliff diver, not worried about splash based scoring will usually clasps hand together. In both cases, the goal is to prevent the arms separating on impact.

 

[tony873004]

i don't see what the surface tension of the water has to do with the orientation of the diver a millisecond before touching the water. the belly-flop happens because, for whatever reason, the diver or jumper was unable to get into a sufficiently vertical orientation to begin with or to maintain the vertical orientation on the way down...

Fair enough... You have to hit with your belly for it to be a belly flop. I was just referring to the flop part. You can slap at the surface of water with your hand and experience a stinging sensation as if you'd slapped the wall, provided your hand is above water to begin with. But once your hand is underwater, you can't slap it anymore. I think it is the surface tension of the water that causes this, and prevents slamming into the surface of the water from feeling like slapping a pillow.

I heard once that in some diving competitions, that air bubbles are released on the bottom of the water to break up the surface tension as they rise to the top. Perhaps someone here who dives can confirm or deny this.

 

[Loren Booda]

rbj,

I appreciate your stating physics that should have been obvious to me. I got the figures from a (now) questionable website.

 

[rcgldr]

I heard once that in some diving competitions, that air bubbles are released on the bottom of the water to break up the surface tension as they rise to the top. Perhaps someone here who dives can confirm or deny this.

It's a visual thing (see reference below), so divers can spot distance to the water to adjust their entry. Air bubble usage is rare. Sprinker systems are almost always used. However on a dive pushing the envelope, like a 3 1/3 front pike dive, I'm not sure how much visual feedback there is since the diver has only small windows while rotating to see the pool and judge distance, which is why you see more mishaps on the high flip count type dives. As mentioned in my updated post, the high divers from 60 feet or more aren't going for maximum difficulty, just a save entry, especially when the height (and survival) is the sole judging criteria.

Then again, you have maniacs like this guy, from 52meters, 170 feet, with a quadruple flip with 1 1/2 twist in the middle. He over rates just a bit, probably knocked a bit of wind out of him, but he climbs out on his own.



FINA rules for diving pools:

FR 5.3.10 Mechanical surface agitation shall be installed under the diving facilities to aid the divers in their visual perception of the surface of the water. In pools equipped with an underwater bubble machine, the machine should only be used for this purpose if it creates sufficient water agitation when working with a very low pressure; otherwise a horizontal water sprinkler system should only be used.

 

[rbj]

Aerodynamics at the speed that high divers achieve doesn't offer enough resistance to allow a diver to remain vertical control. Almost all high divers going in feet first use one or two controlled front flips with a 1/2 twist at the end, and vary the amount of tuck at the end to end up vertical.

Jeff, i am no professional nor olympic nor any other kind of expert diver (just a swimmer with access to cliffs). but i have certainly gone in the water feet first several times from heights above 10 meters. never have i, nor would i dare, try to "use one or two controlled front flips with a 1/2 twist at the end". nor have i clasp my hands together. if i remember to, i'll lift them up over my head. if i forget, they slap against the water upon impact and sting a bit, with a red rash that lasts about a day.

it doesn't require much aerodynamics to rotate the body away from perfectly vertical since the body is not attached to anything else but the air. if you were to pull your legs out horizontally in front of you (in an "L" shape) during a 15 meter fall, you would impact the water with your legs rotated even higher (like a "V" shape) and your butt would hit before your feet. actually, because of conservation of angular momentum, if you started out vertical ("I" shape) and you pulled your legs out in front 90^o to your torso, it would be "<" shaped and, because of aerodynamics, you would spin slowly to the "L" shape and eventually to the "V" orientation. maybe you wouldn't get to "V" before impact from 10 or 15 meters, but i wouldn't want to experiment. but i dispute that aerodynamics have negligible effect. certainly people have screwed up and gotten into an undesireable orientation and could not get out of it and i know that motion of my arms when falling had some effect on my orientation, enough that i could stay vertical.

 

[tony873004]

It's a visual thing...

Maybe I'm wrong then about the surface tension being broken by bubbles or choppy water or whatever.

 

[rcgldr]

Maybe I'm wrong then about the surface tension being broken by bubbles or choppy water or whatever.

I don't thing surface tension is a big factor. As another factor, it would take a tremendous amount of bubbles to reduce the effective density, as mentioned in another discussion regarding if a boat's water line would be affected by going over a bubbly section of water. I wonder if diving inside the water of a water fall would help (like the cases of barrels over Niagra Falls).

 

[tony873004]

Here's a link confirming what you said about spraying water:
http://juniorengineering.usu.edu/workshops/water/water.php

Another way to reduce surface tension is through physical means. If you have ever done a belly flop off the diving board you should be quite familiar with how water's surface tension effected you. Platform divers feel the same sting even when they land properly. To help reduce surface tension, a hose with a nozzle is positioned to spray water into the pool at the spot where the diver enters the water.

 

[tony873004]

... as mentioned in another discussion regarding if a boat's water line would be affected by going over a bubbly section of water. I wonder if diving inside the water of a water fall would help (like the cases of barrels over Niagra Falls).

I saw a Discovery Channel, or maybe it was Nova episode on why large ships sink, and one theory was giant bubbles that altered the water's buoyancy.

Re: Niagra Falls, I would imagine it would have to help. Is the surface even clearly defined at the bottom of a waterfall? There's got to be a reason the barrel riders choose Niagra, rather than a tall cliff into a lake.

 

[Loren Booda]

Bubbles of methane?