Newtonian Gravitation: Physical Mechanisms

For two point masses, the force magnitude is F = Gm1m2/r² and acts along the line joining their centers. Each body experiences an equal and opposite force, but its acceleration depends on its own mass through F = ma. In extended spherical bodies, external gravity behaves as though mass were concentrated at the center.

Gravity continually accelerates orbiting bodies toward one another. An orbit occurs when sideways velocity and inward gravitational acceleration combine so that a body continuously falls around, rather than directly into, its primary. The inverse-square dependence causes gravitational influence to weaken rapidly with distance but never abruptly vanish.

Planetary motions, lunar motion, laboratory torsion-balance measurements, satellite trajectories, tides, and binary-star orbits agree with Newtonian predictions in regimes where relativistic corrections are negligible. Measured orbital periods and sizes allow masses of planets and stars to be inferred.

An orbit is continuous free fall: inertia carries a body sideways while gravity bends its path inward. Because force decreases as 1/r², circular speed falls with radius around a fixed central mass. Equal and opposite gravitational forces act on both bodies even when their accelerations differ because their masses differ.