We can answer this question with two different perspectives. First perspective to consider it as an object moving in straight line at a uniform speed. As per this we can say that there is no force acting in it, or that resultant of all the forces acting on it is zero. Thus the net force is zero.
The second perspective is to consider the internal mechanism of the car as well as the various forces that power generated by car's engine overcomes to move at the uniform speed in a straight line. With this second perspective we can identify the following forces acting on the car.
- The forces acting on and generated by the engine. When we consider the movements of the engine and other internal mechanism of the car, these are not in a state of uniform motion. It should be note that in addition to generating the net force to keep the car moving, there are many other forces generated by the engine, and there are many other reciprocating, circular and other type of motions which are not in a state of equilibrium. We feel these forces in form of the vibrations generated by the car.
- The wind resistance created by motion of car.
- The friction of the internal mechanism of the car.
- The rolling friction created by movement of tyres.
- Gravitational force acting on the car and the reaction of car to the gravitational force. This ensures that there is no downward movement of the car.
There is no net force on a mercedes travelling at constant speed, along straight line. The speed or costant velocity is not the criteria to determine the force. The changing speed or velocities or directions are caused by the force. The rate of change of velocity is the acceleration. The product of acceleration and mass of the mercedes are the criteria to determine the net force of the mass. The mercedes hower has to maintain its speed lost against the road friction, the ups and downs , the humps, the plane of the car and air resistance. These different situation should be taken care separately by providing required controlled external force with the mechanism with the mercedes .