一旦你得到一个这样的计算式，你对向量场做点积，这和前面这个不一样。
Once you have such a formula, you do the dot product with this vector field, which is not the same as that one.
我们来看它的第一个意义，在我们用两个，不一样的向量相乘之前，我们先看一个向量和它本身的点积。
Well, the first thing it means before we multiply two vectors, let's start multiplying a vector with itself That's probably easier.
可以把这些用向量形式重新写下来，就是梯度向量和位置改变量的点积。
And I can rewrite this in vector form as the gradient dot product the amount by which the position vector has changed.
我们要做的是，沿着曲线的每一点上，取向量场和法向量的点积。
What we will do is just, at every point along the curve, the dot product between the vector field and the normal vector.
这是平面法向量，和沿直线向量的点积。
It's the dot product between the normal vector of a plane and the vector along the line.
角动量：(有参照点)的矢量和粒子线性动量的向量积。
Angular momentum: the vector product of the position vector (from a reference point) and the linear momentum of a particle.
角动量：(有参照点)的矢量和粒子线性动量的向量积。
Angular momentum: the vector product of the position vector (from a reference point) and the linear momentum of a particle.
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