Here's why:

* Deceleration means the object is slowing down.

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* When an object decelerates, its speed decreases, so its magnitude of velocity decreases.

* If the object is moving in a straight line, the direction of its velocity may or may not change depending on the direction of the deceleration.

Example:

Imagine a car driving at 60 mph and applying the brakes. The car's velocity is decreasing as it slows down. The velocity is still in the direction the car is moving, but its magnitude is decreasing.

Important Note:

Deceleration is often used interchangeably with "negative acceleration," but it's technically more accurate to say an object is experiencing acceleration in the opposite direction of its motion when it's slowing down.