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1/ I just stumbled on these pics I took 3 years ago at a public pool showing the waterfall feature. There's some awesome science in this. 
2/ I was amazed to see how a simple iPhone camera can freeze the falling water with enough clarity to see the physics of these sheets of water as they tear and fragment.
3/ The basic part (not the part that amazed me) is that as the water falls it speeds up and therefore the sheets have to get thinner and thinner until they tear. You can see this with the flow from a water tap in your house. The stream gets narrower as it falls, as it speeds up.
4/ Why does it get thinner & thinner? Because the amount of water passing each point in the stream per second must be the same amount that passes every other point in the stream per second, because water isn't being magically made or disappearing between each point in the stream.
5/ We call this the "continuity equation". (We use the continuity equation all throughout physics.) This equation tells us that if the water is going faster, then the stream must have gotten skinnier so that the total amount of flow at each point is still the same.
6/ And because the stream is in free fall, it is getting ever faster, and therefore it is getting ever skinnier. But there is a limit to how skinny it can get: eventually random turbulence in the water and in the surrounding air will cause the thin sheet to rip and tear.
7/ You see this with the water falling from the faucet in your bathroom. It gets skinny and then it fragments into drops. We also see something like this in the cosmos. When the universe was young after the Big Bang, it was filled with smooth, hot plasma with no rips or tears.
8/ But because the universe was expanding, the matter that filled all of space was getting thinner, and eventually the small ripples caused it to rip apart into filaments, which fragmented into galaxies, as this simulation shows. (Credit: Volker Springel, Virgo Consortium)
9/ That was what initially caught my attention and caused me to take the picture. But what I saw after zooming in the photo and enhancing the contrast is what I want to point out now. This is dope. Look at the ripples around the tears in the water!
10/ These are clearly a type of wave that form in the sheet of water around the rips as they form. The water seems to be bunching up around the shorelines of these rips. Water is incompressible, so these waves are places where the sheet is folding, or getting thicker & thinner.
11/ These are NOT ordinary waves like we are used to seeing in water. Ordinary waves are "gravity waves". Gravity is vital to their physics. The wave rises up above the water level, so gravity pulls it back down, which displaces the water in front, causing that to rise up, etc.
12/ In other words, gravity is the "restoring force" that causes water waves to exist. Every type of wave needs a restoring force to bring the high, or dense, part back to average, but then it swings too far the other way and thus it ripples across space as a wave on and on.
13/ It blew my mind when I saw this because this is a sheet of water in FREE FALL, so as far as the water can tell there is no gravity. It's like being in the NASA zero gravity aircraft. The aircraft dives as fast as free fall, so inside the airplane you experience zero gravity.
14/ So, if the water is not experiencing gravity, then gravity cannot be the restoring force that causes these waves in the water around the rips. The only other potential force that could cause these waves is surface tension. Water has very strong surface tension because...
15/ ...because the water molecules are polar. I mean, they are lopsided, with the oxygen hogging the electrons too much so they are e little bit negative while the hydrogen atoms are a little bit positive. This lets nearby molecules cling to each other, creating a sheet of force.
16/ Normally you can't see the effects of this surface tension because the weight of the water (and its inertia) is the dominant driver of water flow, but take away that gravity and make the sheet get thinner and this very weak force suddenly become the dominant one.
17/ This is why we need to do reduced gravity research and why we fly in the NASA reduced gravity aircraft with flow experiments, because the physics is so different when you take away gravity. What will it be like to swim in a swimming pool on the Moon in 1/6 gravity???
18/ The fact that waves form around the rips in the water tells us that surface tension is actually holding the sheet of water together as it falls and gets thinner. It would have fragmented into droplets much higher up, but instead the surface tension kept it as a smooth sheet.
19/ And this surface tension keeps it from completely shattering into droplets the moment the first rip forms. It is not like a pane of glass that shatters. The rip forms a shoreline surrounded by waves, the rip grows, but the surrounding water sheet stays intact. Such beauty!!!
20/ Look how crazy beautiful it gets as the tears get wider, until their shorelines merge into filaments as the only thing that remains, much like the early cosmos fragmenting into filaments, and then the filaments also fragment into drops like the galaxies. Wow!
21/ Hmm, the font changed when I saved it. Let's retry that picture in jpg format and with a smaller font. This is better.
22/22 OK, so I just wanted to share this because if you've got an iPhone, you've got a scientific tool better than 99.99% of humanity ever possessed. We can simply take pictures and see this amazing world in ways our unaided eyes never could. The fabric of reality is amazing.
