The universal gravitational constant G, appearing in Newton's law of gravitation, cannot be derived theoretically -- it must be measured experimentally, and because gravity is so weak compared to every other everyday force, this measurement requires an extraordinarily sensitive apparatus: the Cavendish balance (torsion balance). Two small masses are mounted on a light rod suspended by a thin fibre; two much larger masses are brought close to them, producing a gravitational TORQUE (not a net force) that twists the fibre until it is balanced by the fibre's own elastic restoring torque.
Measuring the equilibrium twist angle, together with the known masses, separation and the fibre's independently-measured torque-per-unit-twist, lets G be calculated from the torque-balance equation, yielding the accepted value G=6.67×10−11 N m^2/kg^2, with dimensional formula [L3M−1T−2]. This same constant then appears in every other gravitational formula in the chapter -- for acceleration due to gravity, potential energy, escape velocity, and satellite motion alike -- so a precise measurement of G is foundational to quantitative gravitational physics.