gravity

The downward acceleration that the Earth gives to any object near its surface, irrespective of mass. Magnitude `g ≈ 9.81 m/s²` at sea level, slightly different elsewhere (lower on a mountain, higher at the poles). In the context of projectile motion, "gravity" is shorthand for "the constant acceleration vector pointing toward the Earth's center, taken as straight down for short flights." The mass-independence is what lets a feather and a hammer fall together in vacuum — and is the experimental seed of general relativity.