By the way, I didn't tell you this but I think you know that I picked my origin to be here and measured y positively to be upwards and I called that 15 meters.

顺便说一下,虽然我没明确指出,但我想你们应该都知道,我把原点取在这里,并规定y的正方向朝上,我设把这里的高度为15米

耶鲁公开课 - 基础物理课程节选