Two objects of different mass fall from the same height in a vacuum (no air resistance). Which hits the ground first?

Galileo's principle (verified famously on the moon by Apollo 15's David Scott dropping a hammer and a feather): in a vacuum, all objects fall with the same acceleration regardless of mass. Acceleration due to gravity on Earth: g = 9.8 m/s² (≈ 32 ft/s²). This is because gravitational force on an object (weight = mg) is proportional to mass, but acceleration = force/mass — the mass cancels: a = F/m = (mg)/m = g. So both objects accelerate at g. In AIR (real world), air resistance affects falling objects, and heavier/denser objects do fall faster (a feather falls slower than a rock because air resistance is more significant relative to feather's weight). Terminal velocity occurs when air resistance equals weight — the object falls at constant velocity. Free fall equations: (1) velocity at time t: v = gt (starting from rest); (2) distance fallen in time t: d = ½gt²; (3) velocity after falling distance d: v = √(2gd). Examples: object falls 1 second → velocity 9.8 m/s, distance 4.9 m; falls 3 seconds → velocity 29.4 m/s, distance 44.1 m. Projectile motion: horizontal motion is independent of vertical motion. An object dropped from a height and one fired horizontally from the same height hit the ground at the same time (assuming flat ground and no air resistance). ASVAB MC tests fundamental physics concepts; precise calculation less common than conceptual understanding.