Acceleration can be negative when an object starts to move from rest. In physics, the sign convention for acceleration is typically defined based on the direction of motion.
When an object starts from rest, it initially has a velocity of zero. If the object then starts moving in the positive direction, but gradually slows down and eventually comes to a stop, the acceleration will be negative. This is because the object is decelerating or slowing down in the positive direction.
In other words, a negative acceleration indicates a decrease in velocity over time. It does not necessarily mean that the object is moving in the opposite direction. It simply means that the object is slowing down or decelerating.
On the other hand, if the object starts from rest and accelerates in the positive direction, the acceleration will be positive, indicating an increase in velocity over time.
It's worth noting that positive and negative signs for acceleration are relative to the chosen coordinate system and direction of motion. The key is to consistently define the positive direction and then interpret the sign of acceleration accordingly.