10 upper floors can't maintain the force needed to crush 100 lower floors because some of its energy is absorbed into each new floor that falls. At best, the upper floors could destroy about half of their own mass. Here's some high school physics that helps explain it. Try it with cinder blocks in your yard. You can't crush two cinder blocks with one unless you really shoot it down on them. It's the same principle with skyscrapers.
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