Fire Apparatus Operations and Hydraulics Practice Test

Nozzle reaction comes from the nozzle having to accelerate water as it leaves the hose. The faster and more water you push out, the greater the momentum change, and by Newton’s third law the water pushes back on the nozzle with an equal and opposite force. When you raise the nozzle pressure, the water is pushed out with a higher exit velocity and often a higher flow rate, so the momentum change increases and the reaction force grows. That’s why increasing nozzle pressure makes the nozzle kick harder.

The idea that nozzle reaction is independent of pressure isn’t accurate because the speed and amount of water leaving the nozzle depend on that pressure. It’s also not solely about discharge diameter; diameter affects velocity and flow, but pressure directly influences both, so the reaction is not determined by diameter alone.