Gravity at Earth's Surface
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Physical Law
The acceleration due to gravity at the surface of Earth is approximately given by:
- $g = 9.8 \ \mathrm N \ \mathrm {kg}^{-1}$
Note that this is equivalent to
- $g = 9.8 \ \mathrm M \ \mathrm s^{-2}$
as an acceleration can be defined as the force per unit mass from Newton's Second Law of Motion.
Proof
From Acceleration Due to Gravity, we have that:
- $g = \dfrac {G M} {r^2}$
where:
- $g$ is the acceleration on the body caused by the gravitational field of Earth
- $M$ is the mass of Earth, approximately $5.9736 \times 10^{24}\,\mathrm {kg}$
- $G$ is the universal gravitational constant, approximately $6.674 \times 10^{-11} \, \mathrm N \, \mathrm m^2 \, \mathrm {kg}^{-2}$
- $r$ is the radius of Earth, approximately $6.371 \times 10^{6}\,\mathrm m$.
Hence the result, by direct numerical calculation.
$\blacksquare$
Note that the value of $r$ is significantly variable, depending on where you are on Earth.
For fuller detail, see Gravity of Earth at Wikipedia.
Sources
- 1966: Isaac Asimov: Understanding Physics ... (previous) ... (next): $\text {I}$: Motion, Sound and Heat: Chapter $2$: Falling Bodies: Free Fall