The SI unit of negative acceleration is the same as the SI unit of acceleration, which is meters per second squared (m/s²).

Here's why:

* Acceleration is the rate of change of velocity.

* Negative acceleration simply means the velocity is decreasing.

* The units for both acceleration and negative acceleration reflect the change in velocity over time.

Think of it like this: If you're moving at 10 m/s and then slow down to 5 m/s in one second, your acceleration is -5 m/s². The negative sign indicates that your velocity is decreasing.