Now, when you look at this there is something quite remarkable and the remarkable thing x - throughout this whole trajectory-- if there is no air draft, - if there is no friction-- is not changing.
你们可以看到,有件事非常值得注意,可以注意到,方向的速度，is，that，the，velocity，in，the，x，direction,在整个轨迹当中-,如果没有空气阻力，也没有摩擦力-,是不会改变的。