Question

When is acceleration not constant?

Answer 1

Acceleration is not constant whenever there's a change in velocity

Explanation:

Acceleration is defined as `{\Delta v}/{\Delta t}`

Whenever there is a change in velocity, either due to a change in speed or a change in direction, there will be non-zero acceleration.

Answer 2

Acceleration is not constant if the net force is not constant.

Explanation:

Imagine a rocket propelled car. It has 3 rockets on the back. All the rockets ignite and forward acceleration is constant at 3g's. Then one of the engines has a malfunction and quits. The acceleration changes to 2 g's. The acceleration was not constant because the force was not constant.

I hope this help,
Steve

P.S. In cases where acceleration is constant at some non-zero value, velocity will be changing.