I didn't know I was going home for lunch,

我差点以为不能回家吃午饭了
because I was not going to proceed

因为我打算在听到正确答案之前
until I got the right answer.

我是不会继续下去的
The right answer is, when you define a potential,

正确的答案是当你定义一个势能时
there is a freedom to add a constant to the definition of the potential.

可以加上任意一个恒量来定义势能
If you look at every theorem, everything I derived,

你看看所有的定理所有推导
including the stuff I've hidden under the board,

包括黑板后面的东西
U1 - U2 is what enters everywhere;

写的都是 U1 - U2
the change in kinetic energy is U1 - U2.

U1 - U2 是动能的变化
Therefore, one has a freedom in defining the potential

因此势能的表达较为灵活
to add a constant,

可以加上一个恒量
and what has happened is that the person working with this potential

当时定义这个势能的人