Which scenario is an example of an unbalanced force acting on an object?

• A. A skydiver falls at constant velocity after reaching terminal speed because drag balances weight ( $F\text{\_drag}=mg$ ).
• B. A box initially at rest begins to accelerate to the right when pushed, so the net force is nonzero ( $F\ne 0$ ).
• C. A book rests on a table without moving because the upward normal force equals the downward weight ( $N=mg$ ).
• D. A puck slides on frictionless ice at constant speed in a straight line because the net force is zero ( $F=0$ ).

Answer: (B)

Verified step by step guidance

1. Understand that an unbalanced force means the net force acting on an object is not zero, causing the object to accelerate according to Newton's Second Law: \(\text{\textbf{F}}_{\text{net}} \neq 0\) and \(\textbf{a} \neq 0\).
2. Analyze each scenario to determine if the forces are balanced or unbalanced:
3. 1. Skydiver at terminal velocity: Drag force equals weight, so \(F_{\text{drag}} = mg\), resulting in zero net force and no acceleration.
4. 2. Box initially at rest pushed to the right: The applied force causes acceleration, so net force is nonzero (\(F \neq 0\)), indicating an unbalanced force.
5. 3. Book resting on a table: Normal force equals weight (\(N = mg\)), forces balance, net force is zero, no acceleration.
6. 4. Puck sliding on frictionless ice at constant speed: No net force (\(F = 0\)), so no acceleration.