Final answer:
The relevant principle being discussed is Bernoulli's principle, which states that as the speed of a fluid increases, its pressure decreases.
Step-by-step explanation:
The relevant principle being discussed when a water jet leaves a fixed nozzle with a velocity of 10 m/s is Bernoulli's principle. Bernoulli's principle states that as the speed of a fluid increases, its pressure decreases. In the case of the water jet, as the water flows through the fixed nozzle, it accelerates and its velocity increases, causing a decrease in pressure according to Bernoulli's principle.