As is to study every phenomena in physics, it always starts from a simplified situation yet strictly defined. When Bernoulli came up with his famous Bernoulli’s equation, his subject is

Bernoulli’s equation is derived from the conservation of energy with three premises: 1. the stream must remain constant; 2. the components are incompressible; 3. friction-less.

The kinetic energy increases at the expense of the fluid pressure, as shown by the difference in height of the two columns of water.

The application of Bernoulli’s equation is enormous. Because we don’t have much experience putting ourselves in a moving stream/liquid, the knowledge and usage of this understanding is not intuitive but once we follow the logical reasoning to deduce, it’s satisfying.

A simple and direct example – when a passenger stands by too close to a high-speed train, it’s very dangerous. Because high speed running train decrease the pressure, pushing the passenger toward the train.

A more sophisticated application is to design and make airplanes. Grasping the Bernoulli principle that high speed low pressure and low speed high pressure. narrowing down the pipe in the head, causing the air streaming faster creates a lower pressure relative to the bottom air particles. Hence the airplane will tilt upward.

On the contrary, in car racing, what’s needed is to push the car down to the ground so it won’t be hurl out of trajectory, The design should be opposite.