In introductory kinematics, what occurs when an object's velocity decreases over time?

• A. The object must be moving in the negative direction.
• B. The object must have a constant velocity.
• C. The object has zero acceleration because it is still moving.
• D. The object has acceleration opposite the direction of its velocity (negative acceleration relative to the chosen positive direction).

Answer: (D)

Verified step by step guidance

1. Understand that velocity is a vector quantity, meaning it has both magnitude and direction.
2. Recognize that when an object's velocity decreases over time, it means the speed is reducing in the direction it is moving.
3. Recall that acceleration is the rate of change of velocity with respect to time, and it can be positive or negative depending on whether velocity is increasing or decreasing.
4. Identify that if velocity decreases, acceleration must be directed opposite to the velocity vector, which is often called negative acceleration or deceleration.
5. Conclude that the object experiences acceleration opposite to its velocity direction, regardless of whether the velocity is positive or negative.