How can a jet of water be powerful enough to cut through steel?
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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.
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