Why it is difficult for a firefighter to hold a hose which ejects a large amount of water at a high velocity?

Answer 1

The answer is very simple and it goes back to a basic law of physics.

We all know that for every action, there is an equal and opposite reaction, right? So with regards to a fire hose, as the water discharges from the nozzle, there is force pushing back called nozzle reaction. The narrowed nozzle is forcing the speed of the water to accelerate.

The higher the pressure, the higher the nozzle reaction, for a given nozzle diameter. Some of the reaction is dissipated in the friction of the hose on the ground, but a lot of it results in a force pushing the nozzle back toward the firefighters.

In short, the amount of force necessary to accelerate that water through the nozzle is about the same as the force pushing the nozzle in the opposite direction from the discharge.

For example, if you were to pump 357 gallons per minute at 60 pounds per square inch, through a nozzle orifice of 1 1/2 inches, you would have nozzle reaction of approximately 205 pounds. That means the firefighters would have to push BACK with that amount of force to hold the nozzle in position. If the same amount of water were pumped at the same pressure through a tip with 1 3/4-inch diameter, the force would be closer to 270 pounds. This is a critical consideration when operating a nozzle from the top of a ladder!

Sudden Nozzle Reactions

The nozzle must also be opened and closed slowly to avoid a much higher nozzle reaction due to the static pressure generated at the pump (there is no friction loss when the water is stopped).

Ref NFPA Report: Test of Heavy Stream Appliances, 1957.

Answer 2

newtons law tells us that "every action has equal and opposite reaction". When water is ejected at high speeds it pushes the hose back. The higher the speed and quantity of water ejected, the higher is the force which pushes back the hose nozzle.

Consider the case of a rifle: when you fire a cartridge, a bullet (often of lead) is accelerated and pushed out of the barrel at a high speeds. The force transferred to the bullet is also the force on the rifle. The rifle is pushed back. This is called the recoil of a gun. The heavier the bullet, and the force used to accelerate it, the higher the recoil.

Similarly: the hose nozzle ejecting a large quantity of water at high speed gives the expelled water force and some of the opposing force is felt by the firefighters holding the nozzle.

Consider that water weighs about 8 pounds per gallon and a selected fire hose nozzle might deliver a stream at about 300 gallons per minute at 50 psi. That means there is a transfer of 2,400 pounds of water from one side of the nozzle to the other during that minute or 40 pounds per second. To this you add the amount of force needed to accelerate the water from the diameter of the hose through the smaller diameter of the nozzle, resulting in possibly hundreds of pounds pushing the nozzle away from its discharge.