Answer
When the truck accelerates, the friction force acting on the box pulls the box in the same direction as the truck's motion. However, it is not a strong enough force to accelerate the box with the same acceleration as the truck. This is why the box slides backward relative to the truck. Relative to the ground, the box actually accelerates in the same direction as the truck, but with a smaller acceleration.
The action-reaction pairs are the friction force between the box and the truck, as well as the normal force between the box and the truck.
Work Step by Step
When the truck accelerates, the friction force acting on the box pulls the box in the same direction as the truck's motion. However, it is not a strong enough force to accelerate the box with the same acceleration as the truck. This is why the box slides backward relative to the truck. Relative to the ground, the box actually accelerates in the same direction as the truck, but with a smaller acceleration.
The action-reaction pairs are the friction force between the box and the truck, as well as the normal force between the box and the truck.
