Here's why:
* Acceleration is a vector: Acceleration has both magnitude (how fast the velocity is changing) and direction.
* Constant acceleration implies constant magnitude: If acceleration is constant, its magnitude (the rate at which velocity changes) must remain the same.
* Change in direction changes magnitude: A change in direction, even if at a constant *rate*, will inherently change the magnitude of the velocity. Think about a car turning a corner at a constant speed - the direction of its velocity is changing, meaning its velocity vector is changing, and therefore its acceleration is not constant.
Example: Imagine a car moving in a circle at a constant speed. It's constantly changing direction, so it's accelerating. However, the magnitude of the acceleration (how fast its speed is changing) is always the same, but the direction of the acceleration is always towards the center of the circle.
In summary: While a change in direction can lead to acceleration, it can't be constant acceleration because the change in direction will always also change the magnitude of the velocity.
