See, there's this cool thing called "gravity" we have here on earth that can provide momentum to objects with potential energy, like a huge-ass upper section of skyscraper, which I would assume also had a huge-ass amount of potential energy. As to conservation of momentum, the earth is a thing that exists. It would provide the opposite force that crushes the building when it hits the ground, since compared to the building, its inertia is massive - tends to be a common trait of planets. [spoiler]Is this bait?[/spoiler] [spoiler]Did I get baited?[/spoiler]