Which of the following is an example of an unbalanced force? a. book sitting on a table
b. car sitting in the garage
c. person on an elevator going down
d. person leaning on a wall

Answer C : person on an elevator going down

Explanation:

We look for a change in state of motion in order to have a net force different from zero in the system. The state of motion doesn't change in any of the examples a, b, or d. But there is a change in the state of motion in case c where initially the person and elevator where in static equilibrium, and later the elevator started going down and accelerating from zero speed to another non zero speed.

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