The reason it comes down is gravity is pulling it.

物体向下运动是因为重力作用
In kinematics,

在运动学中
you don't ask the reason behind anything.

我们不关心现象背后的原因
You simply want to describe things the way they are

你只是单纯描述物体的实际状态
and then dynamics tells you how they changed and why they changed.

而动力学则是解释它们运动的方式和原因的
So, I'm going to illustrate the idea of kinematics

所以我会用很简单的例子
by taking the simplest possible example.

来解释运动学
That's going to be the way I'm going to do everything in this course.

今后我在课上讲其它问题的时候也会这样
I'm going to start with the simplest example

我会从最简单的例子开始
and slowly add on bells and whistles

逐渐增加难度
and make it more and more complicated.

让问题越来越复杂
So, some of you might say, "Well, I've seen this before",

有些同学可能会说哦这个我知道