Or, to put it as one of my readers/questioners did,
Why doesn’t the Space Needle, a top-heavy skyscraper, ever tip over and fall?
This is reasonable, and there’s precedent for this. After all, the Leaning Tower of Pisa was falling over, and needed significant foundation-work just to keep it upright.
Now, when you take something like the Space Needle, it looks like most of the weight is in the top:
And it looks even more significant if you stand at the bottom and look up:
Yet, in actuality, the Space Needle is one of the most stable structures ever, and will be one of the last buildings standing in the event of a disastrous earthquake. How is this possible? It’s pretty simple, and requires just one little physics concept: center of gravity. In a human being, your center of gravity is somewhere in your abdomen. The higher your center of gravity is above the floor, the tougher it is to balance. Riding a unicycle, for instance, is particularly difficult:
But, if you do something to tweak this, to lower the center of gravity, it becomes much easier to balance. The easiest case is if your center of gravity is below your floor. This makes it so easy that even an inanimate playmobile toy, if you put enough mass beneath the unicyclist’s tightrope, will automatically balance:
Well, this is what they did when they built the Space Needle. They build, underground, a foundation so strong and so massive, that its center of gravity is actually beneath the ground! This is actually pretty straightforward to do if you can drill into some bedrock and sturdily anchor there.
Why is this stable? Because if your building starts to tip, its foundation will rise, and therefore its center of gravity will rise. But this can’t happen! Gravity pulls things down, and so Newton’s simple laws (and the conservation of energy) forbid the Space Needle from tipping over so long as it stays anchored to its foundation.
Want to see a simpler example? Simply head out to your local public body of water, and look for a buoy.
Know why these things stay upright? Because their center of gravity is below the surface of the water. If you tipped one, its center of gravity would rise, which is forbidden. Buoys actually look like this:
So the only way to knock one over and have it stay overturned is to detach it from the foundation.
So, to conclude, this is the most important step in building a skyscraper: anchoring it firmly to a solid foundation. So long as the entire structure has its center of gravity below the ground, it will never topple. And that’s why the Space Needle doesn’t fall over, even though it looks like it’s top-heavy.