= Standard gravity
{title2=$g_n$}
<Standard gravity> is the conventional reference <acceleration>
$$
g_n=9.80665\,\mathrm{m\,s^{-2}}
$$
exactly. It supplies a fixed reference for unit conversions and for expressing an <acceleration> as a multiple of $g_n$. It is distinct from the locally measured <gravitational acceleration>, which depends on location and height.<NIST Guide to the SI>[Appendix B.9, Acceleration: standard acceleration of free fall; bold conversion factors are exact]{ref}
For example, $2g_n=19.61330\,\mathrm{m\,s^{-2}}$. If a vertical <force> on a <mass> of $2\,\mathrm{kg}$ is calculated using this reference, $F=mg_n=19.61330\,\mathrm N$. The same calculation using a measured local value $g_{\mathrm{local}}$ gives $F=mg_{\mathrm{local}}$ instead. The reference value's exactness does not remove measurement uncertainty from a real <force> or from the actual local <gravitational field>.
One can measure local <gravitational acceleration> by tracking a body in <free fall> with an optical <interferometer>, taking $z$ positive downward and fitting its vertical position to $z(t)=z_0+v_0t+\tfrac12gt^2$ after instrumental corrections. Such an experiment estimates $g$ at the apparatus; it does not determine the already stipulated number $g_n$.<Gravity laboratory by Istituto Nazionale di Ricerca Metrologica>[Laboratory description, interferometric vertical-trajectory measurement and environmental corrections]{ref}
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