Kepler’s Third Law and Universal Gravitational Constant – Rankers Physics
Topic: Gravitation
Subtopic: Keplers Law

Kepler’s Third Law and Universal Gravitational Constant

Kepler's third law states that square of period of revolution ($T$) of a planet around the sun, is proportional to third power of average distance $r$ between sun and planet, i.e., $T^2 = Kr^3$ here $K$ is constant. If the masses of sun and planet are $M$ and $m$ respectively then as per Newton's law of gravitation force of attraction between them is $F = \frac{GMm}{r^2}$ here $G$ is gravitational constant. The relation between $G$ and $K$ is described as:

(2015)

$GMK = 4\pi^2$
$K = G$
$K = \frac{1}{G}$
$GM = 4\pi^2$

Solution:

We know that the time period of a planet is given by $T^2 = \frac{4\pi^2}{GM}r^3$. Comparing this with $T^2 = Kr^3$, we get $K = \frac{4\pi^2}{GM}$. Rearranging this gives $GMK = 4\pi^2$.

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