Could anyone offer me a little help with understanding why the principles demonstrated in Galileo's (probably fictional, I know) experiment involving two spheres of the same mass from a tower don't apply in the following situation please? As I understand it, the principle is essentially that two objects of unequal mass will accelerate under gravitational force at the same rate, unless something else (like air resistance) is acting upon them - thus the famous piece of footage of the astronaut with the hammer and the feather on the moon... So, take two balloons, fill them both to the same diameter - one with water, one with air, then drop them. They have (essentially) the same aerodynamic properties, yet accelerate at dramatically different rates. Forgive my ignorance, but why?