Galileo's hypothesis of "all objects are affected by gravity" was proven on the moon during an Apollo mission. (when there is no air or drag on an object, all objects are affected equally by gravity). So, here's your FUN Physics lesson for the day:
1) Galileo realized that a falling body picked up speed at a constant rate - in other words, it had constant acceleration.
2) He also made the crucial observation that, if air resistance and buoyancy can be removed all bodies fall with the same acceleration, bodies of different weights dropped together reach the ground at the same time. (Thus, the Hammer and Feather drop experiment here)
3) Galileo also noted that if a ball rolls without interference on a smooth horizontal surface, and friction and air resistance are removed, it will move with constant speed in a fixed direction (OR -its velocity remains constant)
Galileo considered the motion of an object when not subject to interference as its “natural” motion. SO, natural horizontal motion is motion at constant velocity, and natural vertical motion is falling at constant acceleration.
VIDEO: The mythbusters repeated the famous "hammer and feather drop" in a vacuum chamber at NASA. Note how the feather bounces almost the same amount as the hammer upon falling.