Awesome! The ball returns to its initial position. So, both the initial and final positions are $$y=10$$. Therefore, the displacement is $$10-10=0$$.
So, to calculate velocity, calculate the displacement and divide it by the time taken. $$\text{Velocity} = \frac{\text{Displacement}}{\text{Time}}$$. In calculus terms, $$v = \frac{dr}{dt}$$, where $$v$$ is the velocity, $$r$$ is the position and $$t$$ is the time.
If the ball takes $$5 \, \mathrm{s}$$ to return, which is correct?
• $$\mathrm{Velocity} \, = 4 \, \mathrm{m/s}$$, $$\mathrm{Speed} \, = 4 \mathrm{m/s}$$
• $$\mathrm{Velocity} \, = 0 \, \mathrm{m/s}$$, $$\mathrm{Speed} \, = 4 \mathrm{m/s}$$
• $$\mathrm{Velocity} \, = 4 \, \mathrm{m/s}$$, $$\mathrm{Speed} \, = 0 \mathrm{m/s}$$
• $$\mathrm{Velocity} \, = 0 \, \mathrm{m/s}$$, $$\mathrm{Speed} \, = 0 \mathrm{m/s}$$