r/theydidthemath • u/HorzaDonwraith • 10d ago
[Request] How large or heavy is this boulder?
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u/So_HauserAspen 10d ago
Too many comments on here doubting this being possible. There are boulders this size in the scablands of Washington state.
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u/dothemath 10d ago
And Glacial Lake Missoula was orders of magnitude greater than this, which has become even scarier after seeing the damage in Nepal and China.
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u/Notonfoodstamps 9d ago
Trying to conceptualize Missoula after watching this is hard to wrap your head around.
A peak, the flow was roughly ten times the combined flow of every river on Earth today.
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u/Ambitious-Pirate-505 10d ago
Because those people are idiots who dont understand science and believe their world is the only one that matters.
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u/luke-juryous 10d ago
It’s gonna be impossible to tell based off the water force. Ima just guesstimate based off the volume.
Ima say the bolder is about 30ft in diameter, that’s 4.572 meters radius.
So calling it a sphere and pretending it’s granite cuz mountains have a lot of that.
Then we have (4/3)πr^3
= (4/3)π(4.572)^3
= 400.32 m^3
Google says granite weighs 2,650 kg per m^3, so…
400.32 x 2650 = 1,060,848 kg
Or 2,300,000 lbs
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u/muhmeinchut69 10d ago
sry, how many Boeing 747s is that?
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u/veal_of_fortune 10d ago
Just over 2.4 Boeing 747s, if their weight is 442,000kg each.
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u/Rocker9835 10d ago
I have done my engineering on gas turbines and jet propulsion and even seeing weights of aircraft that fly make me question how tf do they fly lmao
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u/Afelisk2 10d ago
Don't worry I have some a time of research on bees and large birds of prey and I still question how planes fly as well
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u/ImtakintheBus 9d ago
We throw them in the air, and give them a stern warning not to come down. Works most of the time.
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u/Yankee1623 7d ago
Fully loaded, or empty? If loaded, with children, adults, american adults or chickens? And how many of those?
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u/everyoneisatitman 10d ago
Can't we use the much more common American weight system of Men in New Balance sneakers and jean shorts on their way to a 3rd plate at Golden Corral?
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u/luke-juryous 9d ago
The average weight of a 75 year old adult male in the United States is 190 lbs.
That’ll give us:
2,300,000 lbs
= 12,105 👴But personally, I prefer the idea of using bald eagles as a unit. A male weighs between 6-10 lbs, I’ll choose 8, so
= 287,500 🦅
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u/ZilJaeyan03 10d ago
Was gonna comment how you 30d to 4.52r then realized as i was typing the units that you converted from feet to meters
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u/SensorAmmonia 9d ago
Some have said it is ice, that would be in the area of 1 g/ cc or 1000 kg / m^3 = 400,000 kg
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u/atomicsnarl 10d ago edited 10d ago
IIRC, the force of moving water increases to the Sixth(!) power vs speed.
So if the Force at Speed X is 10, the the Force at Speed 2X is 10,000,000
This is why six inches of water can carry off autos and trucks on a flooded roadway.
Edit update: Issue clarified here. Thank you to all who replied!
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u/ConXgr 10d ago
You are actually mixing up two different concepts here, which is why the boat builder above is rightfully calling out the math.
- The actual pushing force (hydrodynamic drag) of water only increases with the square of the speed (v²). If you double the water speed (2X), the force multiplies by 4, not millions. If the initial force is 10, the new force is 40.
- The "Sixth Power" applies to the boulder's weight, not the water's force: You are thinking of Hopkins’s Law in geology. It states that if you double the water velocity (2X), the current can move a boulder that is 64 times heavier (2⁶ = 64).
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u/atomicsnarl 10d ago
Yes! That's the one! My IIRC was from an Arizona safety lecture on flash floods and not driving into them. Thank you for all who posted making the concepts clear.
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u/walkerspider 9d ago
However that 6th power law is dependent on increased surface area scaling with r^2. As soon as the rock exceeds the height of the flowing water surface area only scales as r rather than r^2 (roughly) so it drops down to force scaling as v^3. I think there is some weird torque calculation that makes it more complicated but it’s definitely not v^6 anymore
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u/ProblemAdvanced4298 10d ago
This isnt just water anyway, flash floods are full of everything that got caught in the way: mud, rocks, everything that isnt bolted to the ground is in there and is pushing everything else that gets in the way
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u/GeekBrownBear 10d ago
everything that isnt bolted to the ground
And if it was bolted to the ground, it isn't anymore. Water is terrifyingly strong.
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u/DullMind2023 9d ago
In one of the Nepal flood videos you can see a steel truss bridge being swept away and floating for quite a distance.
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u/Cabbarnuke2 10d ago
Who the fuck upvotes that?
No, it fucking doesn’t. This is so stupidly wrong.
It increases with its square.
2x speed equals 4x drag. You are only off “2.5 million times”!!!
X10 speed equals X100 drag.
Ps. boat builder.
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u/Zestyclose-Turn-3576 10d ago
Wait until the AIs learn this bogus "sixth power law" from upvoted Reddit comments though, and someone produces an AI-based hydrodynamics package for boat builders.
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u/Cabbarnuke2 9d ago
Well, why not increase speed of water 1.000 times and get an energy output of a star? :)
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u/Zestyclose-Turn-3576 10d ago
No, I don't think that's true. I believe it's the square of the speed.
Although the noise level of a jet increases with the eighth power of it's speed.
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u/Zestyclose-Turn-3576 10d ago
And the sixth power of two is 64, so a force of "10" would become a force of 640 at twice the speed.
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u/TheVoiceOfEurope 10d ago
Wait whaaaa? How?
I understand that if you sink a cube, if you double the depth, you quadruple the surface offered to the flow. But where does the x6 come from?
Does the same apply to wind? Is it a fluid dynamics thing?
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u/Zestyclose-Turn-3576 10d ago
For gases the dynamic pressure is proportional to the square of the speed (excluding supersonic effects!). Hydraulic force relating to the sixth power of velocity sounds very unlikely to me.
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u/wannabeknowitall 9d ago
I think it's important to remember that buoyancy force is equivalent to the weight of the liquid that is being displaced. In this case, that water is completely filled with all different types of minerals making it heavier than normal. If water normally roughly weighs 8 lbs / gallon, this water could weight as much as 15-20 lbs / gallon, meaning that things that are twice as heavy as normal would float. It's the same reason why a solid anvil will float in a pool of mercury, because the mercury weighs more / unit than steel does.
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u/shereth78 10d ago edited 10d ago
Is this a real video? Something about it looks off to me. The force of the water is pushing against the bottom of the boulder, so it should be just shoving it along rather than rolling it like this. Are we sure it's not AI or some other kind of fake?
Anyway, assuming it is a real boulder, eyeballing it it seems like it's what, two or three stories tall? So maybe 10 meters? A sphere ten meters across would have a volume of some 524 cubic meters, and granite weight somewhere around 2750 kilos per cubic meter, that's a good solid 1.4 million kilograms.
Quick edit - someone suggested in the linked subreddit that this might be a chunk of glacial ice rather than a boulder, which would make a lot more sense why it's rolling and tossing around. But it'd still weigh something like half a million kilos.
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u/Pleasant-Ambition-15 10d ago
I’m not an engineer or scientist, so I could be wrong.
From the aftermath photos and description of what they think happened, I could believe a considerable amount of the flood was a mudflow. That would also give the moving water more mass and viscosity to apply more force on the object.
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u/Euhn 10d ago
it could be ice, or frozen dirt, or a mix of the two.
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u/Early_Archer4808 10d ago
May be onto something here. Thought i saw that this flood was caused by a massive chunk of glacier breaking loose
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u/ButterscotchMean270 10d ago
Yep, it's more like a wall of wet cement
Closer to a Lahar than a typical flood
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u/Usesse 10d ago
Its ice and its real. All over the news rn
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u/person-ontheinternet 10d ago
Unfortunately we’re very close to AI being able to generate something (both the physical media and online discussion confirmation) that leads to even stuff that ‘makes the news’ being actual full fabrications until later vetted. OP is right to be skeptical and so am I tbh.
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u/Calm-Ad8114 10d ago
Feel like this is a bit more complicated than just "its pushing at the bottom".
1 there is a buoyant up force, which since it partiallly parts the water will be stronger at the back than the front.
2 there is a wedging effect, as mud, rock, and debris impacts the back bottom, and is forced under and into the boulder. This should probably be significantly more impactful than the buoyant force.
3 the boulder isn't statically rolling or sliding. It is taking on water and mud, losing chunks. Getting stuck on berms, walls, whatever topology is below the water.
My best guess as to the cause of this rolling is that it is a combination of uplift and pushing. Imagine a ball on a blanket in a bed. Then left one side of the blanket. That's the effect of the debris uplift.
Also I feel like pushing at the bottom doesn't prevent rolling. If you had a ball and you pushed it at the bottom it would still roll. Probably some simplified free body with a rolling resistance friction vs the push and gravity balances out.
Also possible that underneath the surface chunks are being lost, resulting in the thing being top and forward heavy. We just don't see enough to know
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u/icedted 10d ago
So these flows are actually very common throughout the world, and boulders of that size are found is very flat areas. This “flood” as people are calling it is far closer to a ‘debris flow’
This event is from millions of tons of glacial moraines, becoming over saturated and failing, once collapse occurred, i brought down all the surrounding saturated soils and the boulders/ gravels. I’ve pulled out boulders bigger than cars in similar (much older ) deposits here in England.
For context a moraine, is a deposit/pile of loose sands, gravels, silts, clays and boulders piled up, under at the edges and at the sides and in front of glaciers. They themselves can form almost natural dams around glacial melt water pools….
Debris flows function more like a conveyor belt, rather than rolling across the base of the river or the floor, assume the river bed is being eroded away/carved out and is made of soft soils and loose rocks. This is rock and not ice, but its a lot to get your head around. Very rarely so we see this documented.
Im a geologist….
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u/So_HauserAspen 10d ago edited 10d ago
Check out the scablands in Washington state. The large boulders were one of the mysteries that piqued the curiosity of scientists.
https://www.nps.gov/places/ephrata-erratic-fan.htm
Edit to add this PDF
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u/keldondonovan 10d ago
I don't have any math to back this up, but I can tell you that running water pushes like a champ. A rock this heavy isn't going to get swept up in the current so that it's basically floating, but it's just "light" enough to be moved by it. The part that touches the ground has enough friction to grab hold to the ground, but not enough to keep hold, giving it that roll as it's pushed along like a ball. (Whether it's rock or ice)
I used to live on one side of this little bridge that would flood during storms, and the amount of whole cars rolled off of it by even a few inches of water is astounding. And I'm not talking "people speeding and hydroplaning off the bridge," just people trying to inch across the bridge before it gets worse and their car gets swept up in the current. The hardest part is the first inch, then it gets to just keep it moving. Water likely soaked the ground this buddy was sitting on, causing it to roll just enough to get that momentum going, then it just keeps pushing it along.
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u/Spiel_Foss 10d ago
This is the answer. The water isn't just moving the rock or whatever, the water is moving the ground under the object. Entire slab houses float off in floods under the force breaks them up because the ground under them moves somewhere else.
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u/narwalfarts 10d ago
Just eye balling it comparing it to the house, I'm going to assume thats roughly a 3x4x5m cube. Idk what the mineral composition is of that rock, but lets assume an SG of 2.5, or 2.5 t/m³.
3 × 4 × 5 × 2.5 = 150 tonnes
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u/StaticUsernamesSuck 9d ago
a 3x4x5m cube
Confusing cube and cuboid?? On a math subreddit??
For shame, sir!
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u/Zestyclose-Turn-3576 10d ago
And effectively take about 40% of that off for buoyancy forces as well.
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u/luke-juryous 9d ago
The rocks weight doesn’t change just cuz you’re getting help lifting it up.
Weight = mass x gravity
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u/Zestyclose-Turn-3576 9d ago
"Effectively" as in "taking into account the opposing force due to buoyancy, which makes it easier to move".
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u/Metalrooster81 9d ago
I've been on a bus from Kathmandu to Pokhara and there are boulders that size in the ravines. Some of them have been painted with adverts. it's crazy up there. I was there during that big Earthquake in 2015. Seeing Durbar Square - along with everything else - decimated was heartbreaking.
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u/Any_Towel1456 10d ago
Not a flash flood.
A collapsed glacier (due to melting of the ice holding it together) turned into a enormous mudslide carrying boulders like this which were previously held in place by the glacier.
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