So you saw in a striking example of where friction helped us to balance, and now I want to discuss an object that is hanging and that can freely swing due to gravity.
这就是著名的靠，摩擦力平衡的例子，现在我想讨论一个，可以在重力作用下，自由摆动的悬挂物体。
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A child on a swing looked up at the scream and saw something like a small boy rolling and holding and falling fast through the branches of the pine tree.