Final answer:
There is no net, external, unbalanced force acting on a car traveling at a constant speed on a straight road, due to the balance of forces resulting in dynamic equilibrium.
Step-by-step explanation:
If a car is traveling on a perfectly straight road at a constant speed, according to Newton's first law of motion, there is no net external, unbalanced force acting on the car. This is because a net force of zero indicates that an object is either stationary or moving with constant velocity; it is not accelerating. In the situation described, the car is in dynamic equilibrium, with the force of friction between the tires and the road being balanced by the drag force of air resistance, and the car's weight being supported by the normal forces from the road. Therefore, while there are forces at play, they are balanced and result in no net external force.
Figure 9.3 in the referenced material illustrates a similar scenario, where a car is moving at constant velocity and is in dynamic equilibrium. This means the horizontal forces (applied force and air friction) and vertical forces (the car's weight and normal force) will cancel out, leaving no net external force. This concept also applies to a car moving in a straight line at a constant speed on a level surface.