Spring-Scale Force Converter

1687 — Newton's Principia. Sir Isaac Newton publishes Philosophiæ Naturalis Principia Mathematica, stating the second law as "the change of motion is proportional to the motive force impressed." Newton himself did not give force a unit; his law is a proportionality. For the next 200 years, force was expressed in arbitrary local units - pounds, livres, kilogrammes-force - depending on which kingdom you were in.

1873 — The CGS revolution. The British Association for the Advancement of Science (BAAS) adopts the centimetre-gram-second system, defining the dyne as 1 g·cm/s² = 10⁻⁵ N. Dyne, erg (energy), and barye (pressure) become the lingua franca of physics for the next half-century, surviving in oceanography and surface-tension work even today.

1879 — The poundal. Engineer James Thomson proposes the poundal to give the foot-pound-second (fps) system a coherent absolute unit: 1 pdl = 1 lbm·ft/s² = 0.138255 N. The poundal lets imperial engineers write F = m·a without a 32.174 g correction factor. It saw moderate use in British naval and aeronautical work but never beat the pound-force in US practice.

1919 — Sthène & MTS. France adopts the metre-tonne-second system for industrial use, defining the sthène as 1 t·m/s² = 1000 N. The MTS system gave heavy-industry engineers tidy numbers (typical machine forces in single sthènes rather than thousands of newtons). The Soviet Union adopted MTS for the same reason. France abandoned MTS in 1961 when SI took over.

1946-1948 — The Newton. The CIPM in 1946 recommends MKS (metre-kilogram-second) as the basis for a coherent metric system and names the unit of force the "newton." The 9th CGPM in 1948 ratifies the name. Twelve years later the SI is born, and the newton becomes the sole official metric force unit; the kilogram-force (kp) survives in mechanical-engineering legacy literature but is officially deprecated by the CGPM in 1978.

1960s-1990s — Imperial fights back. The US declines to adopt SI mandatorily (Metric Conversion Act, 1975, made it voluntary). Aerospace and structural codes remain in lbf, kip, ksi, and psf. NASA famously loses the Mars Climate Orbiter in 1999 because Lockheed delivered software in lbf-seconds while NASA expected newton-seconds. Sticking to one unit system, the report concludes, would have saved $193 million.

2019 — SI constants redefinition. On 20 May 2019 the SI fixes the Planck constant h, elementary charge e, Boltzmann constant k, and Avogadro's N_A at exact values. The kilogram now derives from h (no more Paris cylinder). Since the newton = kg·m/s², force is now traceable directly to atomic constants. The 21st-century convention is firm: newtons for science and SI engineering, kip for US structural, dyne for surface tension, and a soft kgf on bathroom scales. Sterner rockets are measured in MN, civilisational dams in GN, and an apple still weighs one newton.