A truck is traveling 60mph (Constant Velocity), a baseball shoots out of the back at 60mph. Do you really expect the ball to drop straight down?
The following video from a Japanese TV show should be a good laugh for anyone teaching science, regardless of what area you focus on. This video uses a vehicle traveling at high speeds, combined with a radar gun and a mounted baseball pitching machine. Using creativity and access to a driving test track, they drive the vehicle 100km/Hr in one direction while hurling a baseball 100km/Hr in the opposite direction. From the frame of reference (the earth), the two velocity's vectors are represented with opposite numerical signs, one being positive and the other negative. The net result when adding these vectors is zero velocity relative to the earth and the ball drops straight down. When the slow motion video is shown, it provides a stunning example of net velocity and vector addition and can lead to a discussion of Galilean relativity.