An automobile with a mass of 100kg is moving north on a level straight highway with a constant velocity of 80km/h. What is the unbalanced force acting on the automobile?

Since there is no acceleration (i.e, there is a constant speed in the same direction), the unbalanced or "net" force is zero.

To determine the unbalanced force acting on the automobile, we need to first understand the concept of Newton's first law of motion, which states that an object at rest will stay at rest, and an object in motion will stay in motion with the same velocity, unless acted upon by an external force.

In this case, since the automobile is moving at a constant velocity, we know that the net force acting on the automobile is zero. This means that the forces are balanced; the force propelling the automobile forward is equal to the resistive forces acting against it.

Therefore, the unbalanced force acting on the automobile is zero.