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

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.