So when you put a thumb over a garden hose you reduce the size of the opening and to keep the same flow rate the water goes out faster, right? So why can’t you (or can you) achieve arbitrary speed with that method (something like water jet cutting)? If you move your thumb to let less and less space for water to pass, shouldn’t that increase the speed of the water even more since the flow rate needs to get higher and higher? But that obviously doesn’t happen. So what determines the max speed of water? I assume the water pressure would have to be involved somehow.
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A: The speed of water coming out of a hose is determined by the pressure of the water in the hose and the diameter of the hose. If you put your thumb over the hose, you reduce the diameter of the hose and the water pressure remains the same. This means that the water will come out faster to maintain the same flow rate. The maximum speed of the water is determined by the pressure of the water in the hose. If you reduce the diameter of the hose too much, the water will not be able to come out fast enough to maintain the same flow rate, and the pressure in the hose will increase until the flow rate is equal to the pressure. This is why you can’t achieve arbitrary speeds with this method. The maximum speed is determined by the pressure of the water in the hose.
A: The speed of water coming out of a hose is determined by the pressure of the water in the hose and the diameter of the hose. If you put your thumb over the hose, you reduce the diameter of the hose and the water pressure remains the same. This means that the water will come out faster to maintain the same flow rate. The maximum speed of the water is determined by the pressure of the water in the hose. If you reduce the diameter of the hose too much, the water will not be able to come out fast enough to maintain the same flow rate, and the pressure in the hose will increase until the flow rate is equal to the pressure. This is why you can’t achieve arbitrary speeds with this method. The maximum speed is determined by the pressure of the water in the hose.
A: The speed of water coming out of a hose is determined by the pressure of the water in the hose and the diameter of the hose. If you put your thumb over the hose, you reduce the diameter of the hose and the water pressure remains the same. This means that the water will come out faster to maintain the same flow rate. The maximum speed of the water is determined by the pressure of the water in the hose. If you reduce the diameter of the hose too much, the water will not be able to come out fast enough to maintain the same flow rate, and the pressure in the hose will increase until the flow rate is equal to the pressure. This is why you can’t achieve arbitrary speeds with this method. The maximum speed is determined by the pressure of the water in the hose.
One step that is often missed is that the water doesn’t get faster because you are constricting it, it is getting faster at the point of constriction because it is getting slower elsewhere in the hose. This means that there is less friction between the water and the walls of the hose, decreasing how much energy is lost while the water is flowing. The theoretical maximum speed can be found from the Bernoulli equation*, speed=sqrt(2*Pressure/density)
*Once your speed approaches the speed of sound in water, this stops working, and you need more complicated maths
One step that is often missed is that the water doesn’t get faster because you are constricting it, it is getting faster at the point of constriction because it is getting slower elsewhere in the hose. This means that there is less friction between the water and the walls of the hose, decreasing how much energy is lost while the water is flowing. The theoretical maximum speed can be found from the Bernoulli equation*, speed=sqrt(2*Pressure/density)
*Once your speed approaches the speed of sound in water, this stops working, and you need more complicated maths
One step that is often missed is that the water doesn’t get faster because you are constricting it, it is getting faster at the point of constriction because it is getting slower elsewhere in the hose. This means that there is less friction between the water and the walls of the hose, decreasing how much energy is lost while the water is flowing. The theoretical maximum speed can be found from the Bernoulli equation*, speed=sqrt(2*Pressure/density)
*Once your speed approaches the speed of sound in water, this stops working, and you need more complicated maths
You can’t make energy out of nothing.
When you constrict a nozzle you essentially force the pressure energy in the fluid to be turned into kinetic energy, but you only have so much pressure energy that you can convert, how much depends on your pump, and once you’re out of pressure energy that’s it, if you further constrict the flow you’ll just chocke the pump, i.e. the pump will simply start pumping LESS wate to compensate for the blockage.
And even with an arbitrarily powerful pump, you are still limited to the speed of sound, since a contracting nozzle can neverr accelerate a fluid past it’s speed of sound, you need converging – diverging nozzle for that
You can’t make energy out of nothing.
When you constrict a nozzle you essentially force the pressure energy in the fluid to be turned into kinetic energy, but you only have so much pressure energy that you can convert, how much depends on your pump, and once you’re out of pressure energy that’s it, if you further constrict the flow you’ll just chocke the pump, i.e. the pump will simply start pumping LESS wate to compensate for the blockage.
And even with an arbitrarily powerful pump, you are still limited to the speed of sound, since a contracting nozzle can neverr accelerate a fluid past it’s speed of sound, you need converging – diverging nozzle for that
You can’t make energy out of nothing.
When you constrict a nozzle you essentially force the pressure energy in the fluid to be turned into kinetic energy, but you only have so much pressure energy that you can convert, how much depends on your pump, and once you’re out of pressure energy that’s it, if you further constrict the flow you’ll just chocke the pump, i.e. the pump will simply start pumping LESS wate to compensate for the blockage.
And even with an arbitrarily powerful pump, you are still limited to the speed of sound, since a contracting nozzle can neverr accelerate a fluid past it’s speed of sound, you need converging – diverging nozzle for that
The maximum speed at any particular pressure is the same speed an object would reach falling from the height *h* needed to create to create that pressure due to gravity *g*. The formula is √(2*gh*). Domestic water pressure can be due to a head of water of up to 50 m, i.e., that’s how far the surface of the water in your local supply tank is above the level of your house. Plugging that number and *g*=9.8 m/s^2 into the formula gives 31 m/s or 70 mph.
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