Motion diagrams, drawn to scale, may help in this relative v problem.
Ask them for a picture with locations of the track in each 10 consecutive
seconds and then for the location of the object. The students will have
no choice but to plot locations of the thrown object with respect to
the truck and this will show what you want to convey.
I see a spelling error above, too late to correct while in the reply mode.
It should be truck, not track, of course.