You can't have acceleration without a change in velocity. That's the fundamental definition of acceleration: the rate of change of velocity.

Here's why this is important:

* Acceleration measures how quickly velocity changes. This change can be in magnitude (speed) or direction, or both.

* If velocity isn't changing, then the rate of change of velocity is zero. This means acceleration is zero.

Think about it this way:

* A car speeding up is accelerating because its velocity is increasing.

* A car slowing down is also accelerating because its velocity is decreasing.

* A car turning a corner is accelerating because its direction is changing, even if its speed stays constant.

However, there are situations where it might seem like there's acceleration but there isn't:

* Circular motion at constant speed: An object moving in a circle at a constant speed is constantly changing direction, which means its velocity is changing. Therefore, it is accelerating, even though its speed isn't changing. This is called centripetal acceleration.

Let me know if you have any other questions!