Answer: [tex]v_f=g\frac{t}{2}[/tex]
The ball was thrown at the speed of [tex]v_o[/tex].
Maximum height achieved is [tex]h_{max}[/tex]
Time of flight is t.
Now, the time the ball takes to achieve maximum height = the time taken by ball to fall back = [tex]\frac{t}{2}[/tex]
let us just consider the second half of the flight. At [tex]h_{max}[/tex], the velocity would be zero. let us consider as the initial velocity for the second half of the flight i.e. [tex]v_i=0[/tex]
Using the equation of motion:
[tex]v_f=v_i+at[/tex]
where, [tex]v_f[/tex] is the final velocity, a is the acceleration, t is the time taken.
Because the ball would fall under gravity, hence a=g and time of flight would be t/2
[tex]\Rightarrow v_f=0+g\frac {t}{2}[/tex]