The graph as shown in figure below describes the motion of a ball rebounding from a horizontal surface being released from a point above the surface. Assume that the ball collides each time with the floor in elastically. The quantity represented on the y-axis is the ball's (take upward direction as positive)

We know that for a body thrown up, its displacement is given as $\mathrm{S}=\mathrm{ut}-\frac{1}{2}{\mathrm{gt}}^2$ . So the s-t graph is parabolic downwards. Also the ball collides in elastically, so it will rebound to less height every time as shown in the graph. ${\mathrm{t}}_1,{\mathrm{t}}_2,{\mathrm{t}}_3$ are the instants when the ball collides with ground. Here slope of the s-t graph is suddenly changing from negative to positive. It means velocity before collision is negative (downwards) and after collision is positive (upwards).