Final answer:
Newton's first law, the law of inertia, describes that a car moving with constant velocity has no net external force. When a car is accelerating, Newton's second law indicates a net force applied to the car. Both laws are used to infer the vehicle's motion based on the presence of forces and acceleration.
Step-by-step explanation:
The question is concerned with how Newton's laws of motion apply to vehicles in motion and how to interpret their motion based on these laws. According to Newton's first law, also known as the law of inertia, if a car is moving at a constant velocity, this indicates that there is no net external force acting on it. If the car is motionless or moving without acceleration, the forces such as the frictional force and the horizontal force applied to the car are in equilibrium.
On the other hand, if the car is accelerating, as per Newton's second law which relates to the amount of force applied to the mass of the object and its consequent acceleration, this entails that a net force is being applied to the car in the direction of the acceleration. Consequently, the forces are not balanced, and the car will change its velocity, or accelerate, in the direction of the net force acting upon it.