Answer:
inertia
Explanation:
the mass of the box will want to continue on its straight line motion.
As centripetal force on the wheel treads causes the truck to turn, frictional force between the box and truck bed is insufficient to keep the box turning at the same rate that the truck turns, so relative velocity occurs between box and bed until the centripetal force is increased by contact with the truck side wall farthest from the center of curvature.
