Which example illustrates an object subjected to an unbalanced force according to Newton's first law?

An object is subjected to an unbalanced force when it changes its state of motion, which is a central concept in Newton's first law of motion. In the case of a ball rolling off a table, it experiences an unbalanced force due to gravity once it leaves the edge. This force causes the ball to accelerate downward, representing a clear change in its motion from a horizontal movement to a downward trajectory.

In contrast, a car moving at a constant speed on a highway is experiencing balanced forces; the driving force of the engine equals the resistive forces like friction and air resistance, so there is no change in its motion. Similarly, a book resting on a shelf is in a state of equilibrium with gravitational force balanced by the normal force of the shelf, meaning it does not move. Lastly, while a plane flying at cruising altitude may be in a balanced and steady state, it maintains a constant speed and direction without any unbalanced forces acting on it. Thus, the example of the ball rolling off a table most clearly illustrates an object experiencing an unbalanced force.