Newton's Law of Universal Gravitation

Physical Law
Every particle in the universe attracts every other particle with a force $F$ which is inversely proportional to the square of the distance between them:
 * $\mathbf F \propto \dfrac {m_1 m_2} {r^2}$

The direction of the force on either particle is the same as the direction of the displacement vector to the other particle.

Using vector notation:
 * $\mathbf F \propto \dfrac {m_1 m_2} {r^2} \hat {\mathbf r}$

where $\hat {\mathbf r}$ is the unit vector from one particle to the other.

Also known as Newton's Law of Gravitation.

Gravitational Constant
Thus the equation becomes:
 * $\mathbf F = \dfrac {G m_1 m_2 \mathbf r} {r^3}$

However, the idea behind it may have originated with Robert Hooke.