Final answer:
The force that causes a box to slide forward in a pickup truck during a hard stop is the frictional force between the box and the truck bed.
Step-by-step explanation:
The force that caused the box to slide forward in the back of a pickup truck that comes to a hard stop is the force due to friction between the box and the floor of the truck. Friction is a contact force that opposes the motion or attempted motion between two surfaces in contact. In the context of a stopping vehicle, the frictional force would normally act to keep the box from sliding; however, when the truck comes to a sudden stop, the static friction force between the box and the truck bed may not be sufficient to overcome the inertia of the box, leading the box to slide forward due to the kinetic friction taking over. If the static friction is overwhelmed by the inertia of the box, the box will start to slide forward until either the kinetic friction stops it or it hits a barrier.