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View Full Version : Can a water balloon thrown from a building kill someone?


andrea_green
01-30-2014, 01:41 AM
How can I learn about from what height (if any) could an average-size water balloon kill (or injure?) a person? What forces would need to be taken into consideration?

Darth Nader
01-30-2014, 02:00 AM
Is it frozen?

dasmoocher
01-30-2014, 02:09 AM
I've seen a water balloon tossed from the 16th floor or so cave-in a car windshield. Probably wouldn't have been too good if it had hit a person.

Ambivalid
01-30-2014, 02:15 AM
How can I learn about from what height (if any) could an average-size water balloon kill (or injure?) a person?

Killing and injuring are two very different things. You wouldnt need to be that high up in order to whip a water balloon at someone's head and injure them. As far as killing them, dunno.

andrea_green
01-30-2014, 02:35 AM
How might I calculate the exact force exerted by a water balloon on the body per story?

oh and Darth Nader: Not frozen.

Darth Nader
01-30-2014, 02:51 AM
People die sometimes just from a trip and fall... one blow to the noggin and a bad landing?

Dead.

texasmiss
01-30-2014, 03:39 AM
It would depend on the weight of the balloon. the size of the stories, maybe wind gust also.
try poison its easier lol

si_blakely
01-30-2014, 05:32 AM
One important factor is the elasticity of the balloon. In most cases, I expect the balloon to deform round the skull, burst and transfer less energy to the target. Also, the balloon may deform due to air resistance as it falls, which could limit the terminal velocity.
Given the comment above re a car windshield - the flat surface does not allow the dispersal of energy round and past the impact, although it will be over a wider area - this could break the windshield.

kayaker
01-30-2014, 07:14 AM
So, we eventually determine the exact height which maximizes the likelihood a water-filled balloon will kill someone. Later today I read a breaking news story about a man killed by a water balloon dropped from that height. Coincidence?:eek:

CalMeacham
01-30-2014, 07:50 AM
When I was an undergrad, a group of guys on my floor in the dorm constructed a slingshot from the bole of a small tree and about 20 feet of surgical rubber tubing, with a washcloth at the midpoint for holding the load. They shot water balloons with it, and could throw them the length of a city block (without the balloon breaking upon launch).

One day they launched it at a dorm window across the courtyard, a mere 20 feet or so away. Thwe balloon punched a perfectly round hole through the glass, kind of like the way cartoon characters going through glass sometimes leave a perfect outline of themselves instead of shattering it.

That's a pretty impressive concentration of force and momentum. I can't help but think that a person on the receiving end of that would sustain an injury. If you hit them just right, maybe you could kill them.

Machine Elf
01-30-2014, 08:16 AM
How can I learn about from what height (if any) could an average-size water balloon kill (or injure?) a person? What forces would need to be taken into consideration?

Disregarding the material of the balloon itself, you can calculate the pressure of impact based on the water's speed and density. The speed can be calculated from the height of the drop. The total force of impact can be calculated based on pressure and contact area. The size of the balloon will affect both the contact area and the duration of the impact, but it won't really affect the pressure (unless the balloon material is extremely strong).

IMHO, a large balloon dropped from more than a few feet can cause serious injury. As evidence, consider this balloon dropped from four stories onto a porta-potty. (http://youtube.com/watch?v=NNpmPvQVbSw&feature=player_detailpage#t=63) Imagine you're walking by when that hits your head. It won't cave your skull in directly, but it may break your neck and/or knock your head violently to the pavement - which would cave your skull in.

Clearly that's not an "average" sized water balloon, but you can see the potential for grievous injury even from smaller ones.

jasg
01-30-2014, 11:45 AM
Just reverse the question - "From how high would you like to fall into still water?" (No diving or toes first)

The surface area impacted might be less with a water balloon, but the pain becomes more personal...

wolfpup
01-30-2014, 01:21 PM
Just reverse the question - "From how high would you like to fall into still water?"

Not the same at all. Skydivers learn that hitting water at terminal velocity is essentially the same as hitting solid ground, whereas a water balloon has relatively small momentum relative to a falling person and a tendency to deform and likely burst on impact, so if it hits a smallish object it will tend to shed much of its momentum around the object's perimeter. Not to say that it's not potentially dangerous, but not the same thing at all as falling into water.

iamthewalrus(:3=
01-30-2014, 01:54 PM
Assuming a water balloon holds a half a liter of water and is roughly spherical (and that I did my math right), at sea level the terminal velocity given by this calculator (http://calctool.org/CALC/eng/aerospace/terminal) is about 93 m/s.

The actual terminal velocity of a balloon is going to be less than that since the balloon will deform from a sphere and have a larger cross-sectional area.

Doing a bit of handwaving, that speed would be reached after ~11 seconds of falling, which implies a height of ~600m.

So, if you can't kill someone from the Tokyo Sky Tree, it's not going to help to climb up the Burj Khalifa.

AskNott
01-30-2014, 05:25 PM
From personal experience as a 12-year old boy (52 years ago,) I can tell you that a well-thrown water balloon imparts most of its force before bursting. It really hurts. Consider that a pint of water weighs a wee bit more than a pound. Throwing a balloon bigger than a pint is likely to drench the thrower, not the target.

Dropping a bigger balloon, though, shouldn't be a problem. Now, if you wanted to make a more harmful water balloon, I'd suggest using a bigger balloon. Putting, say, a gallon of water in a balloon that would hold much more would deliver more of the force before bursting.

If mayhem is your goal, though, why not use a jug? Or an anvil? Maybe a piano!

It's always darkest just before the piano lands on you. --Oliver Faltz

ducati
01-30-2014, 05:46 PM
Years ago, I visited some friends in a college dorm. This being an all-girls school, the girls on the second floor would climb out their windows onto the roof of the first floor to sunbathe, often in the nude, since no men were around or allowed in the dorms. (I was sneaked by some girls)

One of the girls had the great idea of dropping water balloons on the sunbathing girls below, from the 5th floor. We filled the sink with maybe 20 balloons, and started pitching them out the window without looking.

The screams just made it funnier, until we realized they didn't sound fun. Turns out the roof is covered in gravel, and when the balloons hit, they sprayed the rocks like shrapnel.:eek:


Clearly this was a practice we did not repeat. I would not have wanted to be down there during such an attack!

Penfeather
01-30-2014, 09:25 PM
Did the hardass Dean give you a chewing out and threaten to expel your fraternity from the dance-off?

Habeed
01-30-2014, 09:45 PM
At 93 m/s, 500 grams of water had the energy of 2 162 joules. That's about the same as a 5.56 mm rifle bullet.

The thing is, the energy is spread over enormously more surface area - instead of a tiny bullet tip, it is diffused over half a liter of water. Given a person can survive being shot in the head while wearing a helmet (which spreads out the energy), I think the balloon impact is survivable.

Francis Vaughan
01-30-2014, 11:50 PM
At 93 m/s, 500 grams of water had the energy of 2 162 joules. That's about the same as a 5.56 mm rifle bullet.

The thing is, the energy is spread over enormously more surface area - instead of a tiny bullet tip, it is diffused over half a liter of water. Given a person can survive being shot in the head while wearing a helmet (which spreads out the energy), I think the balloon impact is survivable.

I don't think this quite works. Up until very recently nobody wearing a helmet would survive being shot in the head by a 5.56 rifle. Not unless the bullet has travelled a long way and lost most of its speed. At 93m/s the bullet will not be stopped. Only the new Enhanced Combat Helmet is rated to stop a rifle bullet, all the existing helmet designs are only good for stopping fragments. Until there is some real experience with people being hit, I would be very loath to suggest that such an impact is injury or mortality free.

The other problem with dropped water balloons has been noted earlier. They can produce compression damage at the top of the spine. In this case it doesn't matter what area the balloon spreads over, as the damage is not at the point of impact, but where the force is concentrated in the body. In this case we need an estimate of the peak force rather than the energy.

China Guy
01-31-2014, 12:32 AM
One day they launched it at a dorm window across the courtyard, a mere 20 feet or so away. Thwe balloon punched a perfectly round hole through the glass, kind of like the way cartoon characters going through glass sometimes leave a perfect outline of themselves instead of shattering it. .UC Davis circa 1980?

Silver Fire
01-31-2014, 12:35 AM
Did the hardass Dean give you a chewing out and threaten to expel your fraternity from the dance-off?
... and then the Dean was like, "You're done."

Francis Vaughan
01-31-2014, 02:09 AM
At 93m/s the bullet will not be stopped.

Ugh, unfortunate mix of numbers. The bullet is doing 922m/s, the balloon is doing 93m/s.

Darryl Lict
01-31-2014, 02:25 AM
UC Davis circa 1980?
I lived on the third floor in Tercero at UC Davis in 1976, We rigged up a 2x4 slingshot on the balcony of the third floor and tossed water balloons the better part of a city block. We knocked the sliding glass door of the building across the way, but didn't break it.

I tossed a water balloon at the trams visiting the dorms on Picnic day. It ll hit some poor woman in the side of the head and I saw the recoil. Christ, I felt awful as I had never really thought about the mass of hardball sized water balloon.

usedtobe
01-31-2014, 02:34 AM
The port-a-potti incident was not only an over-sized "balloon", it was not a balloon in the sense of the toy balloon kids know - notice it stayed spherical, whereas a toy balloon would have flopped and deformed.

I suspect this would be a strength-of-materials issue as much as it is a kinetic energy issue. A toy balloon will burst if handled incorrectly - which is a huge benefit for the target.

Putting a liter of water in a cannon ball vs cling wrap, as it were - the issue is how soon the weight is going to run off the skull (let's assume you can hit your target) - is it going to stay long enough to deliver 100% of its energy down the spine, or will it feel like a quick slap while providing an sloppy shower?
I'm leading toward the latter.

Habeed
01-31-2014, 03:11 AM
Only the new Enhanced Combat Helmet is rated to stop a rifle bullet, all the existing helmet designs are only good for stopping fragments.

Not true. The older kevlar helmets were able to stop bullets from some angles.

CalMeacham
01-31-2014, 07:30 AM
UC Davis circa 1980?

MIT circa 1975.

leahcim
01-31-2014, 07:38 AM
These scenarios are all assuming that your target isn't the Wicked Witch of the West. If it is, lethal distance is much, much shorter.

CalMeacham
01-31-2014, 08:14 AM
These scenarios are all assuming that your target isn't the Wicked Witch of the West. If it is, lethal distance is much, much shorter.

Not to mention the Triffids (1963 movie version only), the ant-like aliens in Jack Chalker's Well of Souls series, and Tim Benzedrino.

AaronX
01-31-2014, 10:13 AM
It's sort of like getting hit by a paintball, right? I bet getting hit by a water balloon moving as fast as a paintball would cause injury.

Drunky Smurf
01-31-2014, 11:15 AM
Not to mention the Triffids (1963 movie version only), the ant-like aliens in Jack Chalker's Well of Souls series, and Tim Benzedrino.

Or the aliens from Signs.

dasmoocher
01-31-2014, 12:45 PM
Or the aliens from Signs.

You'd probably need a really big balloon for the Sandworms on Arrakis.

CalMeacham
01-31-2014, 12:52 PM
You'd probably need a really big balloon for the Sandworms on Arrakis.

...and then step back to avoid being drowned in the Water of Life. Whether you're worthy or not.

iamthewalrus(:3=
01-31-2014, 02:10 PM
It's sort of like getting hit by a paintball, right? I bet getting hit by a water balloon moving as fast as a paintball would cause injury.That's a good comparison. The muzzle velocity of a paintball is ~90 m/s. A paintball at point blank range will often raise a welt and may break the skin.

A water balloon probably has 50-100 times the mass of a paintball, and would do some serious damage at that speed. It certainly could break a bone or even kill someone if it hit them in the head.

si_blakely
01-31-2014, 08:58 PM
But a paintball is far more rigid and has a smaller contact area. You can't just extrapolate from that.

j666
01-31-2014, 09:50 PM
How can I learn about from what height (if any) could an average-size water balloon kill (or injure?) a person? What forces would need to be taken into consideration?If you hit them right over the heart immediately after the left ventricle contracts but before it has time to relax, it takes very little force.

Francis Vaughan
01-31-2014, 10:57 PM
If you hit them right over the heart immediately after the left ventricle contracts but before it has time to relax, it takes very little force.

Which does remind me, back in my student days, the surgical rubber water bomb launchers were used on our equivalent of rag day. One year a woman was hit in the chest by one and had a significant cardiac episode. She didn't die, but it wasn't good. I think this subsequently caused the banning of these overgrown slingshots.

Rick
01-31-2014, 11:02 PM
When I was an undergrad, a group of guys on my floor in the dorm constructed a slingshot from the bole of a small tree and about 20 feet of surgical rubber tubing, with a washcloth at the midpoint for holding the load. They shot water balloons with it, and could throw them the length of a city block (without the balloon breaking upon launch).

One day they launched it at a dorm window across the courtyard, a mere 20 feet or so away. Thwe balloon punched a perfectly round hole through the glass, kind of like the way cartoon characters going through glass sometimes leave a perfect outline of themselves instead of shattering it.

That's a pretty impressive concentration of force and momentum. I can't help but think that a person on the receiving end of that would sustain an injury. If you hit them just right, maybe you could kill them.
Another data point.
When I was an undergrad they had "Warrior day" at college where we all fucked off for the day.
Several of us got together about 60 feet of surgical rubber tubing and a dog food dish. Drilled some holes in the dish and strung three ten foot rubbers on each side. Had two guy hold the rubbers and one guy to walk backwards 20-30 feet, hold the dish just right and let fly. Some of the balloons broke on launch, but most didn't. We had fights with several other teams, but due to our superior firepower (3 rubbers instead of two) and our superior balloon holder we could out range the other teams, we were shooting well over 100 yards.
Anyway in the afternoon, they staged a raft race in the reflecting pond, about the size of a football field. We were set up on one side of the pond shooting at teams on the other side. Due to the short range, we were letting fly pretty much horizontally. We were shooting over the heads of the seated raft paddlers.
Anyway I pulled back about 20 feet and let fly at our rivals on the other side. Right as I let go, a guy in a raft right next to our side of the pool stood up directly in front my speeding water balloon. He took it right on the side of the face. He was maybe 10 feet from the point the balloon left the dish. The balloon did not break, but it cleaned that guy right off the raft. :D
Didn't kill him, but he had one hell of a bruise.

AaronX
02-01-2014, 09:55 PM
But a paintball is far more rigid and has a smaller contact area. You can't just extrapolate from that.


Are you saying smaller water balloons do more damage?

Leo Bloom
02-02-2014, 09:55 AM
Rag day?

Accidental Martyr
02-02-2014, 12:57 PM
Rag day?

I had no idea what that was either. Apparently it's related to this:
http://en.wikipedia.org/wiki/Rag_(student_society)

lisiate
02-02-2014, 05:38 PM
Assuming a water balloon holds a half a liter of water and is roughly spherical (and that I did my math right), at sea level the terminal velocity given by this calculator (http://calctool.org/CALC/eng/aerospace/terminal) is about 93 m/s.

The actual terminal velocity of a balloon is going to be less than that since the balloon will deform from a sphere and have a larger cross-sectional area.

Doing a bit of handwaving, that speed would be reached after ~11 seconds of falling, which implies a height of ~600m.

So, if you can't kill someone from the Tokyo Sky Tree, it's not going to help to climb up the Burj Khalifa.

If that calculator is correct then the terminal velocity of the water balloon is 208 mph.

Want to get hit by something 4 times heavier than a baseball going twice as fast as the fastest pitcher on Earth?

Francis Vaughan
02-02-2014, 10:10 PM
Thinking some more about this, and trying to avoid getting into the really difficult world of modelling the fluid, there are some high school physics level things we can think about.

Energy is probably not the right quantity to think about. Energy transfer from the balloon to the head of the victim is going to be very difficult to work out. Energy is subject to transfer to a range of forms, so for instance a bullet hitting a helmet may well have most of its energy dissipated as heat in the helmet, and not imparted to the victim as kinetic energy. The ECH most certainly depends upon such a mode. However momentum is much more useful. Conservation of momentum gets us much less equivocal results.


So, some back of the envelope calculations. (Worth exactly what you paid for them. :D )

If we assume that the water drop hits square onto the top of the victim's head, and the water splashes out in a pretty much horizontal spray, and we assume negligible contribution to the forces from viscosity, friction or the energy needed to rupture the balloon, we can simply look at the momentum change in the drop. This is probably a good assumption for any high speed impact, which are those we are worried about.

It seems that for any sensible balloon size 100m/s is about the terminal velocity (although it requires an unrealistic height to achieve) so that is a limiting case.

We can also assume a cube of water rather than a sphere in order to make a few other simplifications.

So: momentum = mass * velocity. = m * v = p
Impulse = change in momentum * time = p * t
time = velocity / height of cube = v / h = t
mass = height of cube^3 = density of water * h3 = 1000kg/m3 * h3 = m

Impulse = h3 * v * 1000 * (v/h)
= h2 * v2 * 1000

So, a limiting case. A 1 litre cube of water falling at 100ms-1. This has a momentum of 100kgms-1 and takes 0.001s to travel its own length. Thus the force imparted as it splashes is:

100,000 Newtons for 1 ms.

That will certainly fracture a lot of vertebrae and I would find it hard to believe is in any way survivable.

However we might notice that the terms are not linear, and things get more reasonable fairly quickly.
100ms-1 is just silly anyway. So maybe drop it from a 10 story building, call it 30 metres. Final velocity is 24ms-1. Close enough to be one quarter terminal velocity, and so our force drops 16 fold to about 6,200 Newtons over 4 ms. That is still a very serious force, and would likely injure someone, possibly badly.

Final velocity goes up as the square root of the height of the building, whereas the force of the balloon goes up as the square of the velocity, so usefully the force goes up linearly with the height of the drop. So, for a 1 litre balloon about 500 Newtons per metre of drop height. Note, for low drop heights most of the above assumptions don't hold, and the force will be substantially less. From a height of 1 metre the balloon does not explode in a horizontal spray of water, indeed the balloon might not even rupture, and if it does, the water runs down over the victim, transferring momentum over a long period of time.

The force falls slower than the mass falls, because the height of the cube of water falls, and that means the momentum changes over a shorter time.

We can also look at the velocity of the victim. Assume a 100kg victim. (A bit chubby, but it makes things easier.) Hit by a 100kgms-1 balloon, if we assume they are a rigid body, they will end up falling at 1 ms-1. Just imparted to their head, (assume 5kg mass) this momentum would result in their head being accelerated to 20ms-1 (44 miles/hour) onto their neck. I think we can assume this will break their neck. Our ten story building drop gets us a head travelling at 11 miles an hour, which is likely to cause significant injury, but probably not kill them. The momentum goes up as the square root of the height (ignoring atmospheric drag), so even a 5 story drop is likely to be within the realm of causing neck injury. Below about a 5 story drop I susepct the main assumptions start to fail, and the forces start to fall much quicker.

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