core433

Why do replies to this kind of questions always either miss the point entirely or go off on some tangent about stuff we already know? Here's my theory. The OP and his girlfriend are both wrong. The former brings up the empirically correct observation but justifies it poorly. The latter blindly stuck to an elementary fact that was spoon-fed to her and didn't consider that there can be multiple forces acting on an object in this complex universe. Let's state what we know. 1) All objects accelerate equally in an ideal vacuum. 2) A paper, cardboard, and steel sheet of the exact same shape have the same surface area. 3) A steel square falls faster than a cardboard square falls faster than a paper square in air. Saying all 3 will fall at the same rate in air is stupid because empirically we can prove this is not true. Saying air resistance as a function of surface area is the only opposing force to gravity is wrong because all 3 have the same surface area. What I feel can only be the proper answer is therefore aerodynamics, which dictates how an object deforms or changes orientation as it is falling, which dictates turbulence of air which affects its rate of fall. A paper sheet bends like crazy in air due to being structurally incapable of maintaining a stable form in the face of pressure from air, thus moving in a sinusoidal pattern and creating some amount of lift in its travel. Its speed is a function of both surface air resistance and lift. A cardboard sheet exhibits similar behavior but to a much lesser extent. A steel sheet exhibits no such behavior, thus it creates no lift due to aerodynamics and its slowdown is purely due to air resistance. Structural integrity is often correlated with mass, so the OP attributed this phenomenon to mass, which makes him wrong. That is to say, if I take a material that is both light and structurally sound - such as a sheet of carbon nanotubes - and dropped it, assuming it did not deform in air, it would fall at the rate more similar to a heavy object of the same shape than a light, malleable object. This is difficult to envision intuitively because such material rarely occurs naturally.