The Law of Gravitation

Of Newton's accomplishments, the discovery of the universal law of gravitation ranks as one of the greatest. Imagine two masses, $M_1$ and $M_2$, separated by a distance $r$. The force has the magnitude

$$F = \frac{M_1 M_2 G}{r^2} ,$$ (14.1)

where $G = 6.67 \times 10^{-11} \mbox{ m}^3 \mbox{ kg}^{-1} \mbox{ s}^{-2}$ is the universal gravitational constant. The gravitational force is always attractive and it acts along the line of centers between the two masses.