The title of this photo-essay is a little obscure, but very enlightening once you see the photos:
The lesson: if you overload a crane, it tips over. You see this a lot on YouTube:
Sometimes the moment affects the arm rather than the base:
So what is happening here? A moment in physics is a twisting force. So when you put a wrench on a bolt and turn it, you create a moment. The wrench provides a moment arm. Any lever, or even a teeter-totter, is doing the same kind of thing. The longer the moment arm, the bigger the moment.
One way to simplify things is to think about these cranes that tip over as big teeter-totters. The outrigger is the fulcrum for the teeter-totter. On one side, usually the shorter side, is the mass of the crane itself - its body, engine, axles, etc. Think of this (in most cases) as a fat kid on a teeter-totter - you want this side of the teeter-totter to alwats be on the ground. On the other side is the arm of the crane and whatever object it is lifting. If the weight on the arm is too great (and it doesn't take much weight if the arm is long), the teeter-totter tips and the crane falls over.
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