It comes from the fact that velocity is a vector and you can change your velocity vector by changing your direction.

其原理就是,速度是矢量,你可以通过改变方向来改变速度

耶鲁公开课 - 基础物理课程节选

The fact that when you go in a circle, you accelerate is what we're learning here, coming from the fact that velocity is a vector and its change can be due to change in the magnitude or change in direction.

而当你做圆周运动时你也在加速,这是我们在这里所学到的,原因就在于,速度是一个矢量,其变化可以通过改变模长,或者方向来实现

耶鲁公开课 - 基础物理课程节选

If you do, the unit vectors are called i prime and j prime, the components of the vector change.

如果这么做,单位矢量就是 i' 和 j',矢量的两个分量也会发生变化

耶鲁公开课 - 基础物理课程节选