Question

According to which theorem/principle the sum of pressure head, velocity head and gravity head is constant:

Answer 1

1. Pressure head refers to the pressure energy per unit mass of the fluid, velocity head refers to the Kinetic energy per unit mass of the fluid and gravity head refers to the potential energy per unit mass of the fluid.
2. According to Bernoulli’s theorem, the total energy(pressure head, velocity head and gravity head) per unit mass remains constant for the streamlined flow of an ideal liquid at every cross-section throughout the flow.

Thus, in Bernoulli's theorem the sum of pressure head, velocity head and gravity head is constant.

Hence, Bernoulli's theorem is the correct theorem for the given conditions.