It was my favourite equation from school. It made sense; you could push something heavy and it would take a lot of force and if you pushed something light it would accelerate quickly. It was common sense, f=ma; you knew it made sense. But how can this feeling of certainty be equivalent to the feeling of falling bodies? The thought of a light object falling at the same rate as a lead weight doesn't instantly appeal to the psyche. It's not something that feels intuitively right. It isn't the same feeling of certainty that the moving force brings. The force of my hand can be seen to apply to all the particles of a solid object. If gravity is actually due to particles, then it is not necessarily the same case with falling bodies. We can't see the interactions taking place.
http://en.wikipedia.org/wiki/Equivalence_principle
http://en.wikipedia.org/wiki/Equivalence_principle