Question

when do you say an object has positive and negative acceleration.
Urgent......

Answer 1

Positive acceleration=​ A bus or car moving with increasing speed has positive acceleration. We can say that it is accelerating with the direction of travel.
Negative acceleration=​In the above example, if we apply the break, the speed of the car decreases. Then it has negative acceleration. So we can say that it is decelerating.

Answer 2

when ball is thrown up it accquires negative acceleration and when ball come down because of its speed it accquires positive acceleration