Just ignoring the air resistance issues here, I'd like to focus on a more basic aspect of physics that sheds some light on what other posters have taken for granted. Of course, if you're studied physics, this won't be anything new.

If the same force is applied to two different objects, they won't necessarily move with the same acceleration, because different objects have different resistances to acceleration. This resistance to acceleration is, of course, mass.

With that in mind, consider the acceleration one object, of **electic charge** q and mass m, will feel due to an electrostatic interaction with another object of electic charge Q, with a distance r joining their centres. Classically this force is given by the Coulomb force,

F = k q Q / r^2

And by Newton's second law,

a = (q / m) * (k Q / r^2)

where a denotes the acceleration of the first object, which is what we wanted to find. Notice how this depends both on its charge and its mass.

Gravity is very similar to electostatic interactions, and the strength of the interaction is based on what I will call an object's **gravitational charge**. So, we consider the same two objects above, but we consider the acceleration due to gravity rather than electrostatics: the only difference is we replace the q and Q by the object's gravitational charges, which I will denote x and X respectively, so the gravitational acceleration of the first object is given by the same formula as above except we replace the *electric charges* by the *gravitational charges* (and we select a different proportionality constant, G rather than k), giving:

a = (x / m) * (G X / r^2)

If we take r to be the radius of Earth, and X to be the gravitational charge of Earth, then the whole second bracketed part is a constant so I introduce g = G X / r^2 (which turns out to be the familiar 9.81... m/s^2), and write

a = (x / m) * g

So you can see the acceleration due to gravity of an object near the surface of Earth depends both on the object's mass (m) and its gravitational charge (x). But it turns out, experimentally, that an object's gravitational charge is exactly its mass, so x / m = 1 and a = g, which is constant.

This is pretty obvious to anybody who has studied any physics, but I wanted to highlight that the reason the acceleration due to gravity near the surface of Earth doesn't depend on any properties of the object being accelerated is that it turns out *gravitational charge is mass*. In that sense, gravitation is unlike any real force, and in some sense, it's not a force at all, but rather a "correction" that must be made in Newton's laws to account for working in a non-Euclidean geometry.