Final answer:
When an object's acceleration is directed anti-parallel to its velocity, it is slowing down. This situation represents a case of deceleration, or negative acceleration, as the velocity decreases over time.
Step-by-step explanation:
If the acceleration vector of an object is directed anti-parallel to the velocity vector, the object is slowing down. This is because acceleration is in the direction of the change in velocity, not necessarily in the direction of the velocity itself. When acceleration and velocity are in opposite directions, the speed of the object decreases over time, which is commonly referred to as deceleration or negative acceleration in physics terminology.
Considering a coordinate system where motion to the right along the x-axis is positive and to the left is negative, an object with positive velocity and negative acceleration (or vice versa) is undergoing this deceleration process. For example, if a car is moving to the right (positive direction) and experiences acceleration to the left (negative direction), it will slow down. In contrast, an object speeds up when its acceleration is in the same direction as its velocity.