Relation between gravity and Earth’s density – Rankers Physics
Topic: Gravitation
Subtopic: Acceleration Due to Gravity and its variation

Relation between gravity and Earth’s density

The acceleration due to gravity $g$ and mean density of the earth $\rho$ are related by which of the following relations? (where $G$ is the gravitational constant and $R$ is the radius of the earth.):

(1995)

$\rho = \frac{3g}{4\pi GR}$
$\rho = \frac{3g}{4\pi GR^3}$
$\rho = \frac{4\pi gR^2}{3G}$
$\rho = \frac{4\pi gR^3}{3G}$

Solution:

We know $g = \frac{GM}{R^2}$ and $M = \frac{4}{3}\pi R^3 \rho$. Substituting $M$, we get $g = \frac{G}{R^2} \times \frac{4}{3}\pi R^3 \rho = \frac{4}{3}\pi G \rho R$. Rearranging for density gives $\rho = \frac{3g}{4\pi GR}$.

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