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View Full Version : Jumping into shallow water-- safe to land on a matress?

vinniepaz
03-12-2010, 05:11 PM
In an episode of Burn Notice, they jump into a pool from a balcony by throwing a mattress down first and jumping onto that, claiming that it helps to distribute the body weight. It sounds possible, but it also sounds like it could be garbage.

Would this be a smart move in an emergency?

Chronos
03-12-2010, 08:07 PM
The big difficulty with jumping into water is that you have to move a bunch of water out of the way to accommodate you. Jumping onto a mattress would just mean you'd have to move even more water out of the way. I fail to see the benefit.

Superfluous Parentheses
03-12-2010, 08:50 PM
A mattress is a LOT softer than water. I'm not sure about the effects with regards to surface tension, but it you're not jumping from extreme hights, I wouldn't be suprised if it would help.

Harmonious Discord
03-12-2010, 09:31 PM
The thing is if the mattress sets in the water the interior will fill with water. How can the mattress be softer than water when saturated and filled with water? You would have to land on the mattress before it filled with water or any benefits would be lost. Be quick about it.

Noel Prosequi
03-12-2010, 10:41 PM
I would think that too many variables come into play to answer the question definitively. The depth of the pool and the height from which the jump was taken would be the major ones. At what height is one going to dangerously bottom out, for any given depth of pool? Not easy to calculate.

And while the idea of hitting the mattress having the effect of distributing the load so that bottoming out ceases to be a problem is plausible, hitting the mattress itself is not without risk, as is hitting it in a way that is imperfectly centred or unstable, so that one is propelled sideways into the pool's concrete surrounds, etc.

I suspect that the probably multidimensional chart that displays the zones of optimality necessary to determine at what combination of circumstances one is better off using a mattress in this way is, again probably, not something that can be conjured up when your room is on fire or an assassin is breaking through the door.

StrangeBird
03-12-2010, 11:36 PM
sorry, wasn't paying attention - wrong thread

Sage Rat
03-12-2010, 11:47 PM
Just to bring up logistics, but it should also be noted that you have to hit the mattress butt-first. If you hit it with your feet, it's almost certainly going to slide across the surface in one direction or another so that you topple over to the side and hit water or the concrete edge of the pool with your head. But, to actually get out over the pool and land rump-first, you'll have probably have needed to have jumped a good 20 feet since they don't really build pools with overhanging decks above them. Despite what you might think, after one or two feet in mid-air, you're pretty much going straight down.

postcards
03-13-2010, 10:32 AM
My friend Brian Temple
He thought he could make it
So from the third story he jumped

He missed the swimming pool
Only by inches
And everyone said he was drunk

-Lyle Lovett 'Family Reserve'

BigT
03-13-2010, 10:38 AM
The big difficulty with jumping into water is that you have to move a bunch of water out of the way to accommodate you. Jumping onto a mattress would just mean you'd have to move even more water out of the way. I fail to see the benefit.

I thought it was less how much water you moved out of the way, and more the force of impact. I mean, if you jump in from the diving board, wouldn't you still be displacing the same amount of water?

I think the idea is that the jumping on mattress displaces more water, but thus does so at a lower force, and thus you might survive the impact. Unfortunately, I don't remember enough about fluid dynamic to know if that is true.

LSLGuy
03-13-2010, 12:02 PM
If you jump from, say, 20 feet, onto concrete you get hurt. Why? It doesn't move when you hit it, so you slow down too rapidly & the forces exceed the strength of your bones.

If you jump from, say, 20 feet, into deep water you really can't get hurt. Why not? The water gives enough when you hit that you decelerate more gradually and so even though the total energy to be absorbed is the same, it's spread out over enough time that it doesn't exceed the strength of your bones.

If you jump from, say, 20 feet, onto a matress floating on the water, you're landing on something which will provide a lot more resistance than the water. Trying to push 30+ square feet of floating stuff under water means the water will push back a lot harder than the couple square feet of cross-section you have alone. In other words, it will give less. And so the deceleration will be more. The surface doesn't move nearly as much when you hit it, so you're increasingly likely to get hurt.

Hence, my belief is that for deep water, adding the matress is a net increase in danger. In fact, I'd bet that the max no-injury jump heights for typical jumpers are around 10 feet for concrete, 15 feet for a floating matress, and 60 feet for water. In other words, adding a matress makes the water almost concrete-like.

Now for jumping into shallow water ...

You just have to slow down enough to avoid injury when you hit the bottom. You don't have to slow enough to avoid touching the bottom. For any height below the max safe matress jumping height of 15 feet, a typical pool is already deep enough that jumping directly into the water from there is safe.

As a teen I often jumped off the roof of a 2-story house into a pool. It was 25 to 30 feet from roofline to water surface. Touched bottom a time or two in 6' of water, but it was far short of producing injury. Had I gone head first it mighta been worse.

My bottom line opinion: using a matress is a Hollywood BS plan that will get people hurt.

LSLGuy
03-13-2010, 12:09 PM
Belated edit: There might be a narrow range of jump heights, say 10-15 feet, which when coupled with a 2-3' pool depth, might make the matress safer. But it's gonna be a narrow window.

Oh year, ref Sage Rat's point: You can only project outward about as far as you can broad jump. 5-ish feet. A running start might get yuo 10. Our pool needed about a 6 foot horizontal jump to make teh water. Had a neighbor kid come within a fraction of a inch ofnot making it one day.

Quartz
03-13-2010, 12:47 PM
If you hit it with your feet, it's almost certainly going to slide across the surface in one direction or another

That's all to the good as it's still absorbing your impact - from where do you think the energy to slide comes?

drachillix
03-13-2010, 01:16 PM
Belated edit: There might be a narrow range of jump heights, say 10-15 feet, which when coupled with a 2-3' pool depth, might make the matress safer. But it's gonna be a narrow window.

Oh year, ref Sage Rat's point: You can only project outward about as far as you can broad jump. 5-ish feet. A running start might get yuo 10. Our pool needed about a 6 foot horizontal jump to make teh water. Had a neighbor kid come within a fraction of a inch ofnot making it one day.

Its IMHO going to work best with a long fall/shallow water combination. If your average pool is say 5-6 feet deep in the middle falling from a 3rd floor is probably going to result in a very unpleasant hard stop against the bottom of the pool with minimal braking supplied by the water. The mattress makes sure you do not pass clean through the water surface while still providing alot more give than hitting bottom.

Chronos
03-13-2010, 01:23 PM
For that matter, when jumping onto a solid surface, you want to deliberately try to hit it with your feet and then rotate to horizontal, since that way you spread out your deceleration over the maximum distance.

Digital Stimulus
03-13-2010, 01:34 PM
First, I have little idea what a realistic estimation of the physics involved is.

Second, although I don't recall that Burn Notice episode all that well, I seem to remember the situation was that the mattress was thrown and immediately followed down. Sort of like a jumping leap to land on a sled (for downhill snow sledding). But also different, for a variety of reasons. Umm...yeah. Perhaps a better analogue would be from the Bourne Identity, where Jason Bourne holds a bad guy under him while plummeting down a stairwell.

Whatever...I hope y'all get the point, part of which is that a waterlogged mattress isn't really possible in this scenario, as it would only hit the water seconds (at most) prior to being landed upon.

Now, I'd question the following (reiterating my ignorance regarding the physics of this):
If you jump from, say, 20 feet, onto a matress floating on the water, you're landing on something which will provide a lot more resistance than the water. Trying to push 30+ square feet of floating stuff under water means the water will push back a lot harder than the couple square feet of cross-section you have alone. In other words, it will give less. And so the deceleration will be more. The surface doesn't move nearly as much when you hit it, so you're increasingly likely to get hurt.
Now, I realize that you were talking about deep water here, which isn't the situation, but consider:

I think the idea is akin to using kevlar/ceramic layered plates for body armor. That is, the force is dissipated over a larger area (the mattress surface, rather than body surface), therefore reducing the per square inch energy absorption. Rather than, as you say above, "pushing 30+ square feet of floating stuff under water", the intent is to use that 30+ square feet to further cushion the 15+ square feet of human-body surface coming to an abrupt stop.

On preview: pretty much what drachillix said. Long fall/shallow water.

vinniepaz
03-13-2010, 01:36 PM
thanks for the responses.

in the show, the two spies land sort of leaned back on their asses, almost on their backs. of course, the civilian they are protecting misses the center of the mattress, slips off and breaks his foot on the bottom. One problem I see is that I don't think the mattress would "give" much in the water because it has such a large, flat surface. Landing the way they did on the show seems like it could easily knock the wind out of you or worse.

Sage Rat
03-13-2010, 02:17 PM
That's all to the good as it's still absorbing your impact - from where do you think the energy to slide comes?