让我们来做第二部分，这是单位圆上的一段圆弧。

Let's do the second part, which is a portion of the unit circle.

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我们在单位圆上。

And we will say, we are on the unit circle.

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我们已经做过的一个例子是，计算四分之一单位圆上的二重积分。

One example that we did, in particular, was to compute the double integral of a quarter of a unit disk.

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In other words, you don't have to be moving actually in a circle to have the acceleration.

也就是说,你不必真的在一个圆上运动以获得相应的加速度

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I look at the plate on my kitchen table, it's not perfectly round, it's got imperfections; but suddenly I start thinking about circles, perfectly round objects.

我看着我餐桌上的盘子,它不是正圆的,它不完美,但是我马上就可以想到圆圈,正圆的东西

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That means, it can only be rattling around in a circle of radius R.

也就是说它只能,限制在一个半径为 R 的圆上运动

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