When a car moves with a constant speed along a straight road, there is no work done on the car by the net force. According to the work-energy theorem, work is the product of force applied to an object and the displacement in the direction of the force. If the car’s speed is constant, there is no acceleration or change in its kinetic energy, implying that the net force acting on the car is zero. In this case, any force, such as friction or air resistance, is balanced by an equal and opposite force from the engine, resulting in no net work being done on the car.