What is the unbalanced force acting on the automobile?

Question

An automobile with a mass of 2000 kg is moving north on a level, straight highway with a constant velocity of 80 km/h. What is the unbalanced force acting on the automobile?

Answer 1

In order to have an unbalanced force, acceleration must be present.  Since your velocity is constant, acceleration is zero.  Therefore, there is no unbalanced force.

Answer 2

Well, the first thing you should do in almost any physics problem is draw your force diagram. Before trying to decide what "unbalanced force you have" try thinking about all forces currently acting on the car. Once you do this, you'll begin to see that you have the force of gravity which is always balanced (on a solid level surface) by the normal force of the road pushing up on the car. After you've crossed out those two forces, what do you have left? The answer would have to be kinetic friction between the tires and the road. hope this helps - MM

 

- sources: University Physics last semester.