playing a symmetric role.

作为一个对称的角色
So, these two masses have to be there.

所以这两个质量必须要在这里
But then what else should I have there,

但是我还需要一些其它的
what are the other variables in the problem? Yes?

问题中还有些什么其它变量请讲
The distance between them.

它们之间的距离
And I don't know what the distance dependence is.

我不知道距离跟什么相关
I'm going to say that it's some function of the distance between them.

我用它们之间的距离的某个函数来表示
So, we don't know what the distance is. All right?

我不知道距离是什么对吧
So, let's find the acceleration of the apple.

让我们来看看苹果的加速度
That happens to be the mass of the Earth,

恰巧是地球的质量
this function, with r calculated to be the radius of the Earth.

这个函数以地球的半径 r 作为参数
Then, you go to the Moon

然后我们考虑月球