By observing the s-t graph, we can determine the sign of acceleration. Recall if the graph is concave upword, the slope is increasing, if it is concave downward, the slope is decreasing, and if the graph is straight line, the slope is constant.
`OA`: Slope is increasing, `v` is increasing and a is positive
`AB`: Slope is constant and `a=0`.
`BC`: Slope is decreasing, `v` is decreasing and a is negative.
`CD`: Slope is increasing, v is increasing and a is positive
`DE`: Slope is constant, `v` is constant and `a=0`.
