In which of the following situations are unbalanced forces acting on an object?

• A. A hockey puck sliding across frictionless ice at constant velocity
• B. A skydiver falling at terminal velocity
• C. A book resting motionless on a table
• D. A car accelerating from rest at a green light

Answer: (D)

Verified step by step guidance

1. Understand the concept of unbalanced forces: Unbalanced forces occur when the net force acting on an object is not zero, causing the object to accelerate or change its motion.
2. Analyze each situation to determine if the forces are balanced or unbalanced:
3. 1. A hockey puck sliding across frictionless ice at constant velocity: Since the velocity is constant and there is no friction, the net force is zero, so forces are balanced.
4. 2. A skydiver falling at terminal velocity: At terminal velocity, the downward gravitational force is balanced by the upward air resistance, so net force is zero and forces are balanced.
5. 3. A book resting motionless on a table: The downward gravitational force is balanced by the upward normal force from the table, so forces are balanced.
6. 4. A car accelerating from rest at a green light: Since the car is accelerating, there must be a net force acting on it, meaning the forces are unbalanced.