Identifying Unbalanced Force Situations

An unbalanced force acts on an object when the net force is not zero. This causes a change in the object's state of motion, meaning it will accelerate (change speed or direction).

Analyzing the Options

• Option 1: A ball rolling on the ground and eventually stopping
  The ball's speed decreases over time until it stops. This change in speed indicates acceleration (specifically, deceleration). The unbalanced forces acting on the ball are primarily friction and air resistance, which oppose its motion.
• Option 2: A car moving with uniform speed on a straight road
  Uniform speed on a straight road means the car's velocity is constant. Therefore, there is no acceleration. This implies the net force acting on the car is zero (balanced forces).
• Option 3: A person standing still on the ground
  A stationary person has zero velocity and zero acceleration. The forces acting on the person (gravity pulling down, normal force from the ground pushing up) are balanced, resulting in a net force of zero.
• Option 4: A book kept on a table
  Similar to the person standing still, a book on a table is stationary. Its velocity and acceleration are zero. The forces on the book (gravity pulling down, normal force from the table pushing up) are balanced.

Conclusion

The situation involving an unbalanced force is when an object's motion changes. The ball rolling and stopping is the only scenario where the object's speed is changing due to opposing forces like friction.