Bernoulli’s theorem obtained directly from a classical mechanics textbook

In this paper we select a small element of fluid of a given mass and follow it as it moves about in space under the influence of the external forces exerted by the rest of the fluid and by gravity, using only ideas contained in any classical mechanics textbook. Then, without the need to consult a fluid mechanics textbook, we obtain the conditions for the energy conservation in a fluid and, from these conditions, we derive the standard form of Bernoulli's theorem.