Which force is required to change the motion of an object?

• A. Gravity
• B. Friction
• C. Net (unbalanced) force
• D. Magnetic force

Answer: (C)

Motion changes only when there is a net unbalanced force acting on the object. If all forces cancel each other out, the object either stays at rest or keeps moving at a constant speed in a straight line. The net force is the sum of all forces acting on the object, and it’s this unbalanced total that causes acceleration (a change in speed or direction). For example, pushing a cart on a surface with friction only changes motion once your push creates a net force that exceeds friction, causing the cart to accelerate in the push direction. Gravity, friction, and magnetic forces are all forces, but what matters for changing motion is whether the overall forces produce a nonzero net force. That’s why the net (unbalanced) force is the key idea.