Explanation :
It is given that, a heavy box is in the back of a truck and the truck is accelerating to the right.
The description is shown in figure 1. Here, the forces that act on both box and the truck are the normal force, the frictional force and their weight mg.
The free body diagram is shown in figure 2. This shows the total forces ( F = ma )acting on the truck. As a result, the truck will move in a forward direction.
Hence, this is the required explanation.