Then he knows it's going to go up, it's going to curve, follow some kind of parabola, then his hands go there to receive it.

所以他就知道,这颗糖会先向上,再向下,沿着某条抛物线运动,然后他就能判断在哪里接住糖

耶鲁公开课 - 基础物理课程节选

That's correct. So her answer was, if it was a full parabola, then we know it would've been at the ground before I set my clock to 0.

她说,如果运动轨迹为一条完整的抛物线,那么它在我们所设定的零时刻之前,就已经落地了

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