Respuesta :

Edufirst
1) Type of  motion: constant acceleration.
 
2) acceleration, a = 9.8 m/s^2

3) time up = time down = 4seconds / 2 = 2 seconds

4) formula = Vf = Vo - a*t^2/2

Vf = 0 => Vo = a*t^2 / 2 = 9.8m/s^2 * (2 s)^2 / 2 = 19.6 m/s

Answer: 19.6 m/s