Question

it is difficult for the Fireman to hold the hose pipe while sprinking the water on a fire why

Answer 1

Every action has an equal and opposite reaction.
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When the fireman holds the hose, it has an opposite reaction according to the third law of motion. This backward and an equal reaction on the fireman makes him unstable and finds it difficult to hold the hose. Thus, due to the opposite reaction of the water with high velocity a fireman finds it difficult to hold the hose.