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

1. Oct 2, 2007

### JimmyJockstrap

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

2. Oct 2, 2007

### HallsofIvy

Staff Emeritus
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.

3. Oct 2, 2007

### 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..

4. Oct 2, 2007

### rewebster

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

5. Oct 2, 2007

### arildno

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

6. Oct 2, 2007

### rewebster

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

7. Oct 2, 2007

### 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.

8. Oct 2, 2007

### 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.

9. Oct 2, 2007

### 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.

10. Oct 2, 2007

### rewebster

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

11. Oct 2, 2007

### Math Jeans

I think you guys are missing the point. He isn't asking how to survive the jump, hes 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?

12. Oct 2, 2007

### 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??

Last edited: Oct 2, 2007
13. Oct 2, 2007

### JimmyJockstrap

mean hypothetically there is this bridge. Say someone tries to murder or maim me when Im 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.

14. Oct 2, 2007

### 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.

15. Oct 2, 2007

### Math Jeans

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.

16. Oct 2, 2007

### rbj

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.

17. Oct 2, 2007

### 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.

18. Oct 2, 2007

### rcgldr

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.

19. Oct 3, 2007

### Loren Booda

Thank you, so noted.

20. Oct 3, 2007

### rbj

better check your numbers, Loren. i dunno 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.

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.

Last edited: Oct 3, 2007