Final answer:
The correct term for the lost portion of the inside of a horizontal pipe is the 'volume.' This concept is important in fluid dynamics for understanding flow rates and the relationship between pipe diameter and velocity.
Step-by-step explanation:
The lost portion of the inside of a horizontal pipe is likely referring to the volume that is no longer occupied by the substance within the pipe, such as water or gas. In this context, the correct answer would be volume. The cross-sectional area of a pipe, important in fluid dynamics and hydraulics, can be found by rearranging the resistance formula for a cylinder, R = PL/A, to solve for A, which is A = PL/R. Understanding the cross-sectional area is imperative when analyzing the flow rate through a pipe, especially in scenarios where the pipe's diameter changes, affecting the velocity of the flow according to the principle of conservation of mass for incompressible fluids.