Friction loss in hose, pipes, and appliances is:

Friction loss in hoses, pipes, and appliances refers to the reduction in pressure that occurs as water flows through these systems. This pressure loss is primarily caused by the friction between the water and the inner surfaces of the hoses, pipes, and appliances, which resists the flow of water. The longer the distance the water travels and the narrower the diameter of the hose or pipe, the greater the friction loss.

Understanding friction loss is crucial for fire apparatus operators, since it affects the operational efficiency and effectiveness of firefighting efforts. If the pressure loss is significant, it can lead to inadequate water supply at the nozzle, which could hinder firefighting capabilities.

The other options, while related to water flow dynamics, do not accurately describe friction loss. For instance, the increase of pressure from nozzle flow doesn't account for the energy lost due to friction, while the result of hose diameter illustrates a contributing factor to friction loss but does not define what friction loss is. The weight of the water in the hose is unrelated to the concept of friction loss itself. Thus, the correct understanding is that friction loss is fundamentally about pressure loss due to friction encountered while the water moves through hoses, pipes, and appliances.