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 Incoming links: Interferometer

g_{n} = 9.80665 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.^[ref]

For example,

2 g_{n} = 19.61330 m s^{- 2}

. If a vertical force on a mass of

2 kg

is calculated using this reference,

F = m g_{n} = 19.61330 N

. The same calculation using a measured local value

g_{local}

gives

F = m g_{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_{0} t + \frac{1}{2} g t^{2}

after instrumental corrections. Such an experiment estimates

g

at the apparatus; it does not determine the already stipulated number

g_{n}