Then you guys know, you just told me, acceleration is -9.8, We call it "minus" because it's accelerating down and up was taken to be the positive direction.

然后你们都知道,你告诉我,加速度是-9.8,我们取负号是因为加速度方向朝下,而正方向为竖直向上

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

For most of us, gravity acts this way, defines a vertical direction very naturally and the blackboard is oriented this way, so very natural to call that x and call that y and line up our axes.

对我们多数人来说,重力作用下,很自然地确定了竖直方向,而我们的黑板又是这个指向的,所以很自然地定出 x 轴和 y 轴,并标出我们的坐标系

耶鲁公开课 - 基础物理课程节选