This document discusses torsion force and its importance in pre-stressed concrete lab courses. It defines torsion force as a twisting force applied to an object by twisting one end. The document outlines objectives about defining torsion force, explaining formulas, and discussing its importance in pre-stressing. Key concepts covered include the torsion formula for solid and hollow circular cross-sections, limitations of the equations, origins of torsion theory, reasons for understanding torsion in pre-stressed concrete, analyzing torsion, and defining pure torsion.