A ball is dropped vertically from a height d above the ground. It hits the ground and bounces up vertically so height $\frac{\mathrm{d}}{2}$. Neglecting subsequent motion and air resistance, its velocity v varies with the height h above the ground is

For the given condition initial height h = d and velocity of the ball is zero. When the ball moves downward its velocity increases and it will be maximum when the ball hits the ground & just after the collision it becomes half and in opposite direction. As the ball moves upward its velocity again decreases and becomes zero at height $\frac{\mathrm{d}}{2}. \mathrm{This} \mathrm{explanation} \mathrm{match} \mathrm{with} \mathrm{graph} \left(1\right).$