1038: "Fountain"

[SirMustapha]

I love the art in the first panel. I don't know much about drawing, but that seems like it's difficult to do well.

Difficult only if you're a less than mediocre artist. But it is good nonetheless.

The next Darwin Awards will be interesting.

[Maxintech]

I think would be a much better fountain to try the idea out with.

That's a bit out of my way, so I'll make do with one. I even have a spare umbrella for it...

[edit] Actually I'm not making do, cos mine's better

Or take the Old Faithful, it arrives every 91 minutes including holidays.

[tillo]

From were I work I can see this, and I can't say the thought never crossed my mind.

[dzamie]

Also, I feel like it should be beret guy doing this.

Ditto, seems like a white beret thing to do. Though, thinking about it, he's more likely to be up at the top already, standing in an upside down umbrella that is floating on nothing but the implausibility of the situation.

I dunno, implausibility by itself shouldn't be enough. But it might work if he's standing on the umbrella to keep himself up, and pulling the umbrella up so that it doesn't fall with his weight. Of course, that only works if beret guy can lift his own weight plus the umbrella's with his arms.

[theShao]

2. If the umbrella could stand it, it would be ripped right out of the guy’s hand.
3. If it wouldn’t be ripped out, it would break his arm. (1500N! That’s more than the biggest monster of a supercar on Top Gear!)

Not quite the case - the guy isn't anchored to the ground so doesn't have to absorb the full force of stopping the water, instead he moves upwards and only has to bear the force of his own body being accelerated to the speed the water's moving at. I think...

[Red Hal]

As fun as that looks, why not attach the fountain to you in the first place? That way you just use the force of the water jet to lift you rather than needing an umbrella.

Oh, wait

Bindun

[TheSavageNorwegian]

Finally! An appropriate place to talk about the unflappable blunt umbrella! That thing will take hurricane-force winds, easy. That silly senz umbrella only works in heavy wind because it deflects the wind, not withstands it. The blunt might be able to handle a few rides on the jet before it too gets shredded.
http://www.bluntumbrellas.com
http://www.youtube.com/watch?v=YOaEShbyXFc

[x-geek]

The Bellagio would be perfect for this. It has water cannons that can shoot up to 500 feet in the air (though the 120 psi, 4" diameter water stream might kill you), and a 13 foot pool depth to break your fall (assuming you can avoid the fountain structures).

[cellocgw]

Remember the MythBusters show on raising a car? They had to remove the engine block to get the total mass down a bit, but then 6 or 8 nozzles powered with a fleet of Fire Dept pumpers got the car way up in the air.

[Wolfkeeper]

I reckon I could more or less do this. You wouldn't use a brolly though.

You need something with adjustable drag, but not too much drag, and it needs to be shaped to automatically pull itself central on the jet.

You would also have to counterweight yourself, so that there's equal weight either side of the fountain.

And a parachute, and probably a deflection plate underneath, because as you go higher, the jet widens and that jet could probably kill you if it hit you directly.

So it's more like a lift thing really. Probably just about doable.

[FarmerJoe]

Remember the MythBusters show on raising a car? They had to remove the engine block to get the total mass down a bit, but then 6 or 8 nozzles powered with a fleet of Fire Dept pumpers got the car way up in the air.

Flooded sewer works as well

http://www.youtube.com/watch?v=EWuy_A5zOC8

I would not THINK of standing near that evil thing.

[spartahawk]

Here's a guy doing it without an umbrella.

http://www.youtube.com/watch?v=YEBUOoVRRQA

[richP]

Fountain about 10m tall at v²=2gh, or about 17m/s out of what appears to be an aperture of roughly 10cm in diameter, or about 75cm². That's 167.5L/s flow, which if brought to a stop would generate 1500N of force. More than enough to lift an adult male, but not very high, as thrust will drop off quickly with altitude.

On the other hand, umbrella withstanding that much force I don't believe in.

10 cm? Where are you getting that from? It's thicker than his head. (Also, he's a stick figure, so it doesn't take too much force to lift him )

You might be an engineer if you've ever assumed a "horse" is a "sphere" in order to make the math work out easier.

[San Fran Sam]

Or put on a thermal suit and stand over Ol' Faithful.

You'd need a strong umbrella though. This seems like a job for the unbreakable umbrella.

Every winter, I get poked in the head while I am walking down the street by one of these. They're called golf umbrellas.

[Wolfkeeper]

You might well be able to do this if you sat in a sphere.

[hordriss]

You might well be able to do this if you sat in a sphere.

I was thinking of the ping pong balls balanced on a hair dryer trick, too. Your weight coupled to the sphere should keep it's orientation stable.
What about sitting on the convex side of a hollow hemisphere, with somoe kind of equally spaced counterweight system which would act to reverse any tilting?

[Wolfkeeper]

The sphere is stable due to Bernouilli effect, but I'm not sure whether that works over a very wide range of speeds or not. Concave shapes like hemispheres may tend to be negatively stable if Bernouilli dominates, but might be positively stable if Newtonian flow is a better approximation.

[Beren]

The problem with the umbrella is that you're only putting the force of the fountain on one side of it. So assuming you have an unshreddable umbrella, and the ability to hold on to it, you're going to shoot up, and turn into the 'beam' of the fountain. Assuming that the water has a force of 1500N, you're going to have an extremely sore ass.

[casualevils]

XKCD, the webcomic with the peculiar ability to precipitate discussions on fluid dynamics.

[whateveries]

Getting the umbrella into the stream without being deflected in a direction other than up would also prove to be a significant challenge, perhaps using a mesh structure (picture a cross between a cheese grater and a spider web made of kevlar) with a a tighter knit toward the middle of the umbrella structure could allow for controlled acceleration, and if designed cleverly the weave could use the force of the water to draw the umbrella to a central location, perhaps by venting in a way that would create a spinning effect, though the venting would need to be designed to allow for the load to be spinning outside of the main thrust of the water without loing the upwards trajectory, also a spring mechanism in the shaft could allow for the sudden uptake uf energy giving the person a better chance of holding on to the umbrella, as far as the descent goes a closing the umbrella a little should reduce the thrust enough to allow the person to control the descent.

Fuck getting to the asteroid belt Obama get NASA onto this. the people want this. Now.

[K^2]

3. If it wouldn’t be ripped out, it would break his arm. (1500N! That’s more than the biggest monster of a supercar on Top Gear!)
4. If he would have monster arms, a monster grip and a monster umbrella, he’d still break his neck after coming down.

You are way off. 1500N is about twice the weight of an adult male. You'd need quite a grip to hold on to that, but it's nothing remotely super-human. And since he won't be raised anywhere near the full height of the fountain, while injury is likely, the fall should be survivable.

[deepthinker]

Fountain is a shot across the Mythbuster's bow. Do you think they'll give Randall credit for the idea when, not if, they bust this myth?

[deepthinker]

Talk about a shot across the Mythbuster's bow!

[pdfitzgibbon]

Haha, kind of like THIS http://www.youtube.com/watch?v=wu9dLanT4Zc

[Sir Lunch-a-lot]

Aaaand... submitted to Mythbusters.

Fountain is a shot across the Mythbuster's bow. Do you think they'll give Randall credit for the idea when, not if, they bust this myth?

I would hope they feature this strip on the show if they try to test this one.

[Wolfkeeper]

Who needs them? Take a water hose and try it yourself with a scale model!!!

[JimsMaher]

Who needs them? Take a water hose and try it yourself with a scale model!!!

All you really need is a super-soaker, a cocktail umbrella, and a subject to launch into the ceiling.
Squirrels are crazy enough to be test pilots.

[whateveries]

Who needs them? Take a water hose and try it yourself with a scale model!!!

All you really need is a super-soaker, a cocktail umbrella, and a subject to launch into the ceiling.
Squirrels are crazy enough to be test pilots.

Wouldnt the average squirrel be...but I ? Oh! I get it US Imperial measurements mean your cocktail umbrella's are much larger. No wonder you guys need to be 21 before you can legally drink.

[jimhsu]

3. If it wouldn’t be ripped out, it would break his arm. (1500N! That’s more than the biggest monster of a supercar on Top Gear!)
4. If he would have monster arms, a monster grip and a monster umbrella, he’d still break his neck after coming down.

You are way off. 1500N is about twice the weight of an adult male. You'd need quite a grip to hold on to that, but it's nothing remotely super-human. And since he won't be raised anywhere near the full height of the fountain, while injury is likely, the fall should be survivable.

1500N should "just" be doable with both arms without sustaining a rotator cuff injury, assuming reasonably good shape. Both arms would also minimize any torsional forces assuming adequate positioning.

"Sources":

http://www.uwhealth.org/files/uwhealth/ ... epair7.pdf

http://ask.metafilter.com/133821/How-mu ... off-a-limb

http://www.jaaos.org/content/18/2/83.full

Thanks for sniping a med student.

[BlitzGirl]

Who needs them? Take a water hose and try it yourself with a scale model!!!

The pool I lifeguarded at had mini water jets and an abundance of small children. But where will I find a scale umbrella?

[The Moomin]

You might well be able to do this if you sat in a sphere.

This?

[nashwaak]

Two things could happen here with a sufficiently sturdy umbrella: either the fountain is directly below the guy, and it lifts him directly by painfully blowing up his ass (making the umbrella just for show), or the umbrella is slightly offset from him, and the resulting torque rips his arm(s) out of their sockets. I'd say neither is a great idea.

A few major errors in the fluid-mechanical analysis people have posted here: the umbrella doesn't just stop the flow, it redirects 20-30% of the momentum back downwards, resulting in a lift about 1.2-1.3 times that that just stopping the flow would give (it has a back-side drag coefficient of about 1.2 or 1.3; air drag on the other side is insignificant). Also, once the guy is moving, the force rapidly decreases due to his relative velocity (with respect to the moving fountain water), not just due to the loss of force with height.

I have to give a fluid mechanics exam in a few days, and I'd like to thank xkcd for giving me a ready-made question, complete with comic, no less! (and also thanks for the Narnia-wardrobe at the bottom of the ocean comic)

[Роберт]

Fantasy:
It works, b*tches?

[JimsMaher]

Wouldnt the average squirrel be...but I ? Oh! I get it US Imperial measurements mean your cocktail umbrella's are much larger. No wonder you guys need to be 21 before you can legally drink.

Squirrels are crazy enough to be test pilots.

[webgiant]

I'm sure people have thought of this, and on the off chance it actually worked, the fall would hurt.

If the umbrella is strong enough to carry him upwards, then it is strong enough to act as a parachute on the way down.

Unless we're saying that its cartoon world physics on the way up, then real world physics on the way down.

[aerion111]

If the umbrella is strong enough to carry him upwards, then it is strong enough to act as a parachute on the way down.

Not necessarily.
Upwards it largely just needs to be big enough to catch as much water as possible, while surviving the force of the fountain.
Down-wards, it would need to have a large enough surface to actually slow you down.

The best way to illustrate might be to imagine a man standing on one of those sewer-covers as water suddenly blows out: If the water has sufficient force and the man can keep the cover balanced, he will be pushed upwards.
When the water-stream stops, however, the man-hole cover will do nothing to slow the man's fall, it itself having a higher density to begin with, and the man being able to create more air-resistance by just lying horizontally spread-eagle like a sky-diver.
So, if the umbrella is too small/heavy, it'll do the person no good on the way down.

[eran_rathan]

Wouldnt the average squirrel be...but I ? Oh! I get it US Imperial measurements mean your cocktail umbrella's are much larger. No wonder you guys need to be 21 before you can legally drink.

Squirrels are crazy enough to be test pilots.

THIS is getting sigg'd.

[Wolfkeeper]

A few major errors in the fluid-mechanical analysis people have posted here: the umbrella doesn't just stop the flow, it redirects 20-30% of the momentum back downwards, resulting in a lift about 1.2-1.3 times that that just stopping the flow would give (it has a back-side drag coefficient of about 1.2 or 1.3

That only works if the jet is very much wider than the umbrella, otherwise it can act as an impulse turbine and redirect more or less all of the momentum back downwards a back-side drag coefficient of about 2.

[Not R]

I got a very different idea from this comic. In the panels with the umbrella open, we no longer see any spray. Maybe this fountain is actually driven by an attractive force from above (with some weird form that makes it stronger at greater distance) rather than pressure below.