Which of the following is the best example of an unbalanced force acting on an object?

• A. A skydiver falling at terminal velocity where air resistance $=$ the force of gravity
• B. A car accelerating forward when the engine applies a force $>$ friction and air resistance
• C. A hockey puck sliding across frictionless ice at constant velocity
• D. A book resting motionless on a table with gravity and the normal force equal in magnitude $=$ and opposite in direction

Answer: (B)

Verified step by step guidance

1. Understand the concept of balanced and unbalanced forces: Balanced forces occur when all forces acting on an object cancel each other out, resulting in no change in the object's motion. Unbalanced forces cause a change in the object's velocity (acceleration).
2. Analyze each option by identifying the forces involved and whether they balance each other: For example, if the forces are equal in magnitude and opposite in direction, they are balanced.
3. For the skydiver falling at terminal velocity, note that air resistance equals the force of gravity, so the forces are balanced and the velocity is constant (no acceleration).
4. For the car accelerating forward, the engine applies a force greater than the opposing forces of friction and air resistance, resulting in a net force and acceleration, which means the forces are unbalanced.
5. For the hockey puck sliding at constant velocity on frictionless ice and the book resting motionless on a table, the forces are balanced because there is no acceleration in either case.