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Newton's Third Law states that for every action, there is an equal and opposite reaction. This fundamental principle of physics means that forces always come in pairs. In our example, the action-reaction pairs can be seen within the system of the truck and the box.
When the truck applies a force on the road through its tires, the road applies an equal and opposite force back on the truck. This force propels the truck forward. Additionally, as the truck bed pushes on the box with static friction to move it forward, the box pushes back on the truck bed with an equal and opposite force. These interactions illustrate Newton's Third Law in action.

It's important to emphasize that while these forces are equal in magnitude and opposite in direction, they do not cancel out because they act on different objects. This is why the truck moves forward while the box tends to slide backward, further illustrating inertia which is the box’s tendency to stay at rest as the truck moves.

Forces are vectors, which means they have both magnitude and direction. In the context of the truck and box scenario, we need to understand several key forces at play to fully grasp the motion involved.
One primary force acting on the truck is friction. The force of friction between the truck's tires and the road enables the truck to accelerate forward when you press the gas pedal. Meanwhile, a force of air resistance acts in the opposite direction, trying to hinder the truck's motion.
For the box on the truck, while at rest, it remains under the influence of static friction, which is the initial resistant force between the truck bed and the box. When the truck accelerates rapidly, inertia causes the box to appear to move backward, even as static friction attempts to prevent this motion. If the force of static friction is overcome, kinetic friction comes into play, causing the box to slide as you observed.
Understanding these forces helps explain why the motion of the truck and box changes when the light turns green, combining the factors of useful friction aiding movement, and resisting forces altering expected paths.

Friction is critical in real-world applications, preventing slipping and enabling controlled motion. There are two main types of friction involved in our truck scenario: static friction and kinetic friction.
Static friction occurs when the box is at rest relative to the truck bed. It is this force that initially tries to "stick" the box to the truck as it accelerates. The strength of static friction depends on the surface materials and the normal force—the force that acts perpendicular to the surfaces in contact.
When static friction is exceeded by an applied force (such as the truck's rapid acceleration), kinetic friction takes over. Kinetic friction is usually less than static friction and acts against the direction of motion, which is why the box begins to slide backward in the truck.
Understanding the role of these friction forces is essential in predicting and controlling motion within systems like vehicles transporting cargo. Knowing how forces interact at various thresholds ensures stability and helps in designing safety features to prevent unwanted sliding of loads.