you go on a circle,

在空中转一个圈
and for awhile you're upside down.

有一会儿你是头冲下的
And the famous question is,

这个著名的问题是
"Why don't you fall down?"

"你为什么没掉下来"
What's the trick?

诀窍是什么
Does it violate Newton's Laws, and so on?

它有没有违背牛顿定律或别的什么
We'll find we can understand it fully with Newton's Laws.

我们发现利用牛顿定律完全可以理解这一现象
But we'll find out that in order for this thing to pull it off,

但我们发现为了确保它不掉下来
its speed at that instant has to have a minimum value.

小车在那一刻的速度有最小值
And the application of newton's law to this object is very simple.

牛顿定律在其中的应用很简单
Here is gravity.

这是重力
What about the track on which it's going?

它所走的这条轨道呢