As we know, either from the film Thor or The Avengers, once Mjolnir is dropped or set aside, it becomes immovable for everyone who is unworthy. What would happen, then, if Thor, while in Hulk's cage, had dropped it? Would it go bouncing all over the place such as Thor or will it be kept fixed in place? And, if so, would you be able to move the cage while the Hammer is still inside?
Mjolnir responds to gravity the same way as any other hunk of metal would in free-fall. The enchantment that prevents one who is unworthy from wielding it would still be in place while you were falling. Grabbing it in free-fall by anyone not Thor or not worthy, would be meaningless.
While falling its position in the cage would continue to move until the cage came to rest. Once on the ground, you would have to cut around it to dig it up. Or in the case of the hammer, dig around it. You would just dig until you couldn't move something and that would be the hammer. Mjolnir would remain immobile once it hit the ground.
The enchantment says he that is worthy. It does not say he that is worthy while in near-zero gravity. In zero gravity, I imagine as long as you aren't trying to use it as a weapon it might be able to be moved, but not wielded. Knowing Odin, Mjolnir would grow spikes or become prickly or covered in lightning in zero gravity to prevent people from trying to touch it.
The hammer is still affected normally by gravity, it has only shown resistance to being moved by sentient beings who are unworthy.
Therefore, it is entirely likely that it would abide by the rules of physics (or at least 'movie physics') while in free-fall.
I believe that it would remain at rest relative to the ground once it landed - the cage would not be movable without being able to move the hammer (although you could cut out a space around the hammer and lift the rest of the cage away.