Final answer:
The work done on a car that accelerates smoothly from a stop sign is positive because the displacement and force applied by the engine are in the same direction, indicating an increase in the car's kinetic energy.
Step-by-step explanation:
When a car accelerates smoothly from a stop sign, the work done on the car by the engine is positive. This is because work is defined as the product of force and displacement in the direction of the force. As the car starts from rest and accelerates, it is being moved in the direction of the applied force from the engine. Since both the displacement and the force are in the same direction, the work done is positive.
According to physics principles, acceleration occurs when there is a change in velocity over time. If a car is speeding up, which means its velocity is increasing, it has positive acceleration. The engine delivers power to the wheels, causing the car to move forward and increase its speed from zero to a certain value, and consequently, the car's kinetic energy increases. The increase in kinetic energy of the car implies that positive work is being done on the car.
It's important to note that the concept of acceleration doesn't only refer to increasing speed; it can also involve deceleration, or slowing down, but in the context of this question where the car is speeding up from a stop sign, the acceleration and work done are positive.