Newton's law of universal gravitation

Any two objects with mass pull on each other. The AP Physics 1 CED describes the force as directly proportional to each of their masses and inversely proportional to the square of the distance between the systems' centers of mass, printed on the equation sheet as $|\vec{F}_g| = G m_1 m_2 / r^2$ with $G = 6.67 \times 10^{-11}\ \text{N} \cdot \text{m}^2/\text{kg}^2$.

Three details decide most questions.

• $r$ runs center to center, not surface to surface. For something resting on the ground, $r$ is Earth's radius, not zero.
• The force is attractive, and it is always exerted along the line connecting the centers of mass of the two interacting systems.
• The dependence is an inverse square. Triple the separation and the force falls to one ninth; halve it and the force goes up fourfold.

Both objects feel the same size of force in opposite directions, however lopsided the masses are. Earth pulls a falling apple with a few newtons and the apple pulls Earth with exactly the same few newtons. Only the accelerations differ, because the masses do.

Close to a planet's surface the expression collapses into weight, $F_g = mg$, since $r$ barely changes over the height of a lab bench. The CED makes that the definition of weight: the gravitational force exerted by an astronomical body on a relatively small nearby object.