But if I see you through the glass, the object is going on some trajectory, which has got both up and down and horizontal motion.

但是如果我透过窗户看到你,这个物体沿着某个轨迹运动,既有上下,也有水平方向的运动

耶鲁公开课 - 基础物理课程节选

You see how long you are in the air and the whole time you are in the air, you are traveling horizontally at this speed.

你可以知道在空中停留的时间,当你在空中运动的时候,你一直以这个水平速度在运动

耶鲁公开课 - 基础物理课程节选

Quite often, it's natural to pick x and y in a certain way, because starting a projectile near the Earth, it makes sense to pick the horizontal as x and vertical as y.

我们经常会很自然地选取 x 轴和 y 轴,因为如果在地表附近做抛物运动,我们通常以水平方向为 x 轴,竖直方向为 y 轴

耶鲁公开课 - 基础物理课程节选