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

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That means, if you are going in a plane and you throw something up in the air, it seems to be going up and down to you.

也就是说,如果你在飞机中,你向空中扔出一物体,它也许会相对于你上下运动

耶鲁公开课 - 基础物理课程节选