Rotational Force

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Rotational Force

Keeping this teetering tower upright involves a lot more than mere luck.

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Experienced Jenga players know that the quickest way to a falling tower is to pull away the two outside pieces of the bottom row, leaving the whole structure balancing on a single narrow wooden block. With only one support at the bottom, every bump and nudge of the tower is magnified, causing it to sway precariously from side to side. But what exactly are the forces that act upon a structure with such a narrow support? And what makes them so dangerous?

Structural engineers don't talk about keeping a building "balanced." They talk about maintaining rotational equilibrium. Imagine a tall building as a long lever arm with the majority of the arm above ground and a smaller section (the foundation) below ground. The point where the building meets the ground is the fulcrum of the lever. Now picture the building tipping slightly to the right or the left. Instead of merely falling over, you can think of it as rotating around the fulcrum. Engineers and physicists have two names for this rotational force: the moment or torque.

A basic tenet of structural engineering is that the longer your lever arm (or the further it is away from the fulcrum), the greater the moment. To decrease the moment of a very tall building, you need to build wide supports. The wider the supports, the lower the moment. To understand this, try standing with your feet spread wide apart and have a friend try to push you over sideways. It requires a lot of force. Put your heels together and try the same thing. Your friend barely has to touch you and you tip right over. A structure with a nice wide base is inherently more stable that a building with a narrow base.

For the last structural engineering lesson learned from Jenga, we'll talk about earthquakes.

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