A free falling mass from an elevation z > 0 (in a vacuum) will reach a speed v = 2 g z , when arriving at elevation z = 0. Or when rearranged as head : h v = v 2 2 g The term ⁠ v / 2 g ⁠ is called the velocity head, expressed as a length measurement. It represents the internal energy of the fluid due to its motion.