Which of the following situations describes an unbalanced force acting on an object?
Correct: C
An unbalanced force causes a change in the state of motion (speed or direction) of an object. A ball rolling to a stop on a grassy field is decelerating due to the force of friction, which is an unbalanced force acting opposite to its motion. In the other cases, the net force is zero (constant velocity or at rest). While a satellite orbiting Earth is constantly changing direction, implying an unbalanced force (gravitational force), its speed is constant (in uniform circular motion), which can make it confusing for this level. The deceleration of a rolling ball is a clearer example of an unbalanced force causing a change in speed.