Newton's Law of Universal Gravitation

Newton's Law of Universal Gravitation states that every particle attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers.

Gravitational force has two key properties:

Gravity is always attractive. Unlike electric forces, gravity only pulls objects together. There is no gravitational repulsion. Two masses $m_1$ and $m_2$ always pull toward each other.

Gravity acts along the line joining the centers of mass. The force on each object points directly toward the center of the other object. For spherical objects like planets, we can treat all mass as concentrated at the center.

The diagram shows two masses separated by distance $r$. Mass $m_1$ experiences force $\vec{F}_{21}$ (force on $m_1$ due to $m_2$) pointing toward $m_2$. Mass $m_2$ experiences force $\vec{F}_{12}$ (force on $m_2$ due to $m_1$) pointing toward $m_1$.

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