newton
Forceexact by definition
The newton is the coherent SI unit of force, defined as the force that accelerates one kilogram at one metre per second squared. Its factor is 1 because it is built from base units with no numerical constant. Since the kilogram, metre and second are all defined from fixed constants, the newton is exact by construction.
| 1 N | 1 N |
Named for Isaac Newton, whose second law it expresses. The name was adopted by the CGPM in 1948.
About the newton
The newton is defined so that \(F = ma\) carries no conversion constant: one newton applied to one kilogram produces one metre per second squared. That coherence is the whole design principle of the SI, and it is what makes the metric system easy to compute in. In the customary system the same equation needs \(g_c\) sitting in it, because the pound-force and the pound-mass were defined independently of each other.
For intuition, one newton is roughly the weight of a small apple, about 102 grams, on Earth's surface. A person of 70 kg presses down with about 687 N. A newton is a small unit for structural work, which is why kilonewtons dominate civil engineering: a 1000 kg load is 9.81 kN, and floor loads are specified in kN/m². Note also that the newton is a derived unit written \(\mathrm{kg\,m\,s^{-2}}\), and that expanding it is often the fastest way to check whether an unfamiliar formula is dimensionally sound.