今天我选择了扶梯，作为静力平衡的道具。

So, today I have chosen a ladder as my subject of static equilibrium.

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现在给你们看一些，静力平衡的例子，不那么凭直觉而定。

Now... I want to show you some examples of static equilibrium which are not so intuitive.

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处于静力平衡的物体，仅仅所有作用力之和，为0是不够的。

For an object to be in static equilibrium, it is not enough that the sum of all forces is zero.

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