How can a jet of water be powerful enough to cut through steel?
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If you look at [this video](https://www.youtube.com/watch?v=sIEqXbXxpfg) you will see a fine sand in the center of the piece being cut. So the water jet is high pressure and very high speed, and has sand in it, which acts as a very strong abrasive.
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Water jets can be made extremely powerful by pressurizing the water and forcing it through a small nozzle. The high pressure of the water causes it to move at a very high velocity, and the nozzle helps to focus the water into a narrow stream. When the water stream hits the surface it is cutting, the impact can be enough to cut through even very tough materials like Sterling. The water jet can also be made more effective by adding abrasive materials to the water stream, which helps to wear away the material being cut. The combination of high pressure, velocity, and abrasive action can make a water jet a very powerful cutting tool.
It isn’t. Water jet cutters have an abrasive material in the water that does the actual cutting. The water just allows the machine to be very fine and accurate.
Water is pretty bloody relentless, have you heard of the grand canyon?
Water, like other liquids, is VERY difficult to compress. In fact, in basic physics classes, we usually just teach that water is completely incompressible, because it takes such enormous amounts of pressure to cause much of any change in volume. Even at the bottom of the ocean, water’s only compressed by a percent or two.
In more quantitative terms, the *bulk modulus* (a measure of compressibility) of water is about 2.2 GPa, which means you need about 22 MPa of pressure (1% of that) to compress water by 1%. For comparison, normal air pressure is about 100 kPa, meaning you need about 220 atmospheres of pressure to compress water by even 1%. That’s a really high bulk modulus; it’s higher even than some soft solids (wiki lists both sandstone and rubber as lower).
In everyday terms, water is very hard, almost as hard as a solid object. You can move around easily in it, but not by compressing the water – you move by moving the water out of the way. If you hit water – or if water hits you – fast enough that the water can’t get out of the way, [you’re in trouble](https://what-if.xkcd.com/93/).
And that’s how water jets work. They shoot water out at high speed, too fast for the water to just squish gently against the surface and splash away. (Actually, even regular water droplets produce a pretty substantial pressure if they’re going at a decent speed, but it’s only for a very brief moment.) A jet sustains the pressure of a high-speed impact constantly, in a way that can easily be aimed and fueled. It’s effectively like pointing a machine gun with really thin bullets at the thing you’re trying to cut.
Think of the difference between a flashlight and a laser. The light we experience in our everyday lives isn’t that harmful, but if you take light and really focus it and power it up, you get a laser beam that can cut through steel. But it’s still just light. The same light in a pleasant, harmless sunbeam can turn into a deadly laser with enough focus.
Same thing applies to water. It’s all about the amount of energy.
Have you thrown a rock at a wall before? You can do it lightly which won’t leave a mark. You can do it hard which may crack the wall. Or if you had a cannon you could launch it and break straight through.
Its the same with water. Throw it at something with enough force (pressure) and it’ll go (or cut) straight through it.
There are a few other things helping it. Water is incompressible, so it doesn’t just squeeze up like a bouncy ball when it hits something hard.
You could think of it as just transfering force to a small area. The actual power comes from whatever motor is pushing the water. The nozzle focussing all the motor’s power in to a very small area which the water transfers to the steel. We just use water because it’s a cheap, non dangerous, abundant, incompressible liquid.
Fun fact: there’s something called sand blasting where you blast sand at a surface to cut off rust/paint etc. Its a very similar process, just using lower pressure so it doesn’t cut and something more abrasive (sand) so it scrapes away the top layer.
You do know that uncompressed, non-jet rain can warp, crack and cut concrete, metals, rock, stone over periods of time right? Mountains have disappeared just from normal rain cycles.
Now imagine if you add pressure (and often times microscopic diamonds for cutting power) and concentrate it at a tiny point.
Very easy to understand.
Take a kiddy ball pit. If you jump into it, the balls float away and you land softly. If i would shoot at you with the same balls at high velocity it would hurt badly. That’s because you can’t compress them, they usually just move out of the way. Same with water. If you don’t give water the chance to move away, it can be very hard. Everyone who jumped into a pool the wrong way knows that.
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