How can a jet of water be powerful enough to cut through steel?
In: 63
REMEMBER! It dug the GRAND CANYON! Water and wind, thats it! Lots of other huge, mammoth, colossal, feats of nature’s…. Water and wind only!
Have you ever seen the Grand Canyon? That is a good example of the power of water.
Yes, waterjet cutting has a number of creative and efficient uses.
As pointed out they often use an added abrasive, frequently crushed garner, it was shocking when I first saw 20 totes, pallet size but a cube of crushed garnet.
I did a project where the customer was using waterjet to trim the flashing off of forged titanium jet engine blades.
But other used were slicing rice-crispy treats ( the very high pressures also sterilize the water), acoustic ceiling tile( they do not even get wet)… granted not steel but some of the variety of applications.
From Newton, F = mA.
F is force. That is what is hitting the steel.
M is mass. Water is heavy. Often it is mixed with something else. Not really important for this part.
A is acceleration, or in this case deacceleration. Say the water is going 1500kph. It hits the steel and immediately goes to 0 (within microseconds), or even bounces off and going negative. That is directly proportional to the force generated. The faster the jet, the more the force. Ex. Say it goes from 1500km/s to -100km/s (bounces off), that is a speed change of 1600 kps, and that that happens in one microsecond.
1500000m/s / .000001s……
That is a big number.
As a wise man once said, “a feather can absolutely knock you down if it’s traveling fast enough”.
In reality though, there’s abrasives mixed into the water. High speed abrasives = cutting.
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.
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.
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.
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.
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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