The bus goes over a bump.Its velocity is initial horizontal but it acquires a small upwards component as it rises, then a downwards component, then back to being purely horizontal.

• 当汽车驶过一个小包时，上坡时原本是水平的速度获得了一个小小的向上的分量，下坡时又有了一个向下的分量，然后回归水平。