When a ball is dropped on a floor, then
$y=\frac{1}{2} gt^{2}$ $\ldots\left(i\right)$
So, the graph between $y$ and $t$ is a parabola. Here $y$ decreases as time increases. When the ball is bounces back then
$y=ut-\frac{1}{2} gt^{2}$ $\ldots\left(ii\right)$
Eq. (ii) is also the form of a general equation of parabola So, the graph between $y$ and $t$ will be a parabola. Here, $y$ increases when time increases. Hence, the required graph between $y$ and $t$ is shown in the figure
