Gravita

Meet Gravita, two masses pulling on each other across empty space.

Isaac Newton published his law of universal gravitation in 1687: every mass attracts every other mass with a force proportional to both masses and inversely proportional to the square of the distance between their centres. Double the distance and the force falls to a quarter. The constant G is extremely small, so the pull between everyday objects is tiny; Henry Cavendish first measured it in a laboratory in 1798.

Earth and the Moon: with masses 5.972 × 10²⁴ kg and 7.342 × 10²² kg, separation 3.844 × 10⁸ m and G = 6.674 × 10⁻¹¹ N·m²/kg², F = 6.674 × 10⁻¹¹ × 5.972 × 10²⁴ × 7.342 × 10²² / (3.844 × 10⁸)² ≈ 1.98 × 10²⁰ N.

The same law explains the tides, planetary orbits, and how space agencies send probes to other planets.

Newton's law is an excellent approximation but not the final word. In very strong gravity or for precise measurements, such as Mercury's orbit or GPS satellites, Einstein's general relativity is needed.