An unbalanced force is needed to change an object's state of inertia

• A. True
• B. False
• C. It depends on mass
• D. Only for moving objects

Answer: (A)

Inertia is the resistance to a change in motion. To change that motion, you must apply a net external force. According to F = m a, if forces on an object are balanced, the net force is zero and there is no acceleration, so the object's velocity stays the same—whether it's at rest or moving with constant speed and direction. Only when a net (unbalanced) force acts does the motion change. So the statement is true: an unbalanced force is needed to change an object's state of inertia. Mass affects how much the motion changes (the acceleration for a given force), not whether a force is needed at all. Starting from rest also requires an unbalanced force to begin moving.