Even though the graph is going up and down, the object is moving from left to right.

虽然图上曲线是上下波动,但是物体是由左向右移动的

耶鲁公开课 - 基础物理课程节选

When you have a velocity in the moving plane frame and you want to find the velocity on the ground, you should add to every object in the plane the velocity of the plane with respect to the ground.

当你在运动的飞机上运动,然后你想找到相对于地面的速度,你就应该给每个物体加上,飞机相对于地面的速度

耶鲁公开课 - 基础物理课程节选