Acceleration-Time Graph Watch
* the ball accelerates with a constant amount of 9.81m/s^2
* as ball bounces up, it has negative acceleration hence -9.81
*as ball moves down, it has positive acceleration , however on the graph it will be on the negative side
because anything moving downward is considered as negative on at graph
So it should be a straight line of -9.81m/s^2 in my opinion. Let's see what others have got to say on it.