Why is it valid to use only $\overrightarrow{F_x}=m\overrightarrow{a_x}$ when analyzing a block whose motion is purely horizontal on a flat surface?

• A. Because weight and normal force always vanish whenever an object moves on a surface
• B. Because Newton's second law applies only to one dimension at a time and forbids vertical analysis
• C. Because horizontal motion means the object has no mass in the vertical direction
• D. Because the vertical forces cancel, giving zero vertical acceleration, so only the horizontal component determines the motion

Answer: (D)