Solution. a. The particle has positive acceleration when velocity is increasing. On the intervals (0, 1 s) and (6 s, 7 s), velocity is increasing--slope is positive--and so acceleration is positive.b. Acceleration is the slope of the v-t graph.The graph has zero slope whenever the graph is horizontal; that is, whenever the velocity is constant. This happens in the intervals (1 s, 2 s), 5 s, 6 s) and (7 s, 8 s).. Acceleration is the slope of the v-t graph. The graph has zero slope whenever the graph is horizontal; that is, whenever the velocity is constant. This happens in the intervals (1 s, 2 s), 5 s, 6 s) and (7 s, 8 s).