Here's why:

* Velocity describes the direction and speed of an object's motion.

* Acceleration describes the rate of change of velocity.

If acceleration is in the opposite direction to the velocity, it acts to decrease the object's speed, causing it to slow down.

Example:

Imagine a car traveling eastward (positive velocity). If the car applies brakes (acceleration in the westward direction, opposite to the velocity), it will slow down.

Important Note: This only describes the initial behavior. If the acceleration continues in the opposite direction, the object will eventually come to a stop and then start moving in the direction of the acceleration.