It's a ubiquitous fact, that if you took any mechanical system

有一个普遍的事实是如果研究某一力学系统
which is in a state of equilibrium,

它处于平衡状态
and you give it a little kick, it vibrates.

你对它施加一个外力它就发生振动
Now, if you've got a pillar, ceiling to floor,

如果现在有一根悬挂在天花板上的柱子
you hit it with a hammer, it vibrates.

你用锤子敲打它它就振动
If you take a gong and hit it, it also vibrates.

如果敲一面锣它也会振动
If you've got a rod hanging form the ceiling through a pivot--

假如有一个通过支点连到天花板上的一根棍
it's hanging there-- if you pull it, it goes back and forth.

它挂在这里如果你拉它它就前后摆动
The standard example everyone likes to use is

一个很常用的例子是
if you've got a particle in a bowl, it's very happy sitting in the bottom.

假如碗里有一个小球它很乐意呆在底部
If you push it up a little bit, it'll go back and forth.

如果把它推高一些它就前后运动
And the example that we're going to consider is the following.

我们要考虑的是下面的例子