Newton's Law of Gravitation

Newton's law of universal gravitation (1687): every two point masses attract along the line joining them with F = Gm₁m₂/r².

G = 6.674 × 10⁻¹¹ N·m²/kg² — the universal gravitational constant.

Force is always ATTRACTIVE, inverse-square in distance, proportional to product of masses.

Gravity acts equally on both bodies (Newton's third law): each pulls the other with the same magnitude F.

For spherically symmetric mass distributions, you can replace the sphere by a point mass at its centre (shell theorem).

Inside a uniform shell: gravitational field is ZERO. Inside a solid sphere of uniform density: F ∝ r (linear).

Cavendish (1798) measured G using a torsion balance — first laboratory test of gravity.

Universal: same law works between two atoms, Earth-Moon, Sun-Planets, galaxy-galaxy. Galileo's free-fall, Kepler's orbits, and tides all follow from this single equation.