Well, you might say

你或许会说
the simplest thing you've been doing all semester long

我们这整个学期研究的最简单的问题
is to take things in one dimension.

不就是一维的情形嘛
But that turns out to be too low.

但一维物体太过于简单
If you're in one dimension and you've got a rigid body--

如果在一维的情形下考查某个刚体
By the way, a rigid body means not only extended but actually rigid.

注意刚体意味着不仅有体积还具有刚性
For example, if you take a cat or a snake,

比如如果你拿一条蛇或者一只猫
a snake you can grab at one point,

你可以握住一条蛇的某一点
but the snake is not simply doing rotations around the one point,

但它不会乖乖地绕着这个点转动
right? Trying to bite you,

对吧它会来咬你
trying to move around

会想法子挣脱
a snake is not a rigid body.

所以蛇不是刚体