Which statement best describes the difference between $\mathrm{positive}$ and $\mathrm{negative}$ acceleration?

• A. $\mathrm{Positive}$ acceleration means an object is moving upward, while $\mathrm{negative}$ acceleration means it is moving downward.
• B. $\mathrm{Positive}$ acceleration always results in an increase in speed, while $\mathrm{negative}$ acceleration always results in a decrease in speed, regardless of direction.
• C. $\mathrm{Positive}$ acceleration is only possible when an object is at rest, while $\mathrm{negative}$ acceleration is only possible when an object is moving.
• D. $\mathrm{Positive}$ acceleration occurs when an object's velocity increases in the direction of motion, while $\mathrm{negative}$ acceleration occurs when the object's velocity decreases in the direction of motion.

Answer: (D)

Verified step by step guidance

1. Understand that acceleration is a vector quantity, meaning it has both magnitude and direction.
2. Recognize that 'positive' and 'negative' acceleration do not inherently mean moving upward or downward; instead, they relate to the direction of acceleration relative to the velocity.
3. Recall that positive acceleration means the acceleration vector points in the same direction as the velocity, causing the object's speed to increase.
4. Understand that negative acceleration (often called deceleration) means the acceleration vector points opposite to the velocity, causing the object's speed to decrease.
5. Conclude that the key difference is whether the acceleration increases or decreases the object's velocity in its direction of motion, not the object's position or whether it is moving up or down.