It is a phenomenon of producing induced current in a coil by changing the magnetic feild around the coil is called Electromagnetic induction.
When magnetic feild is changed uniformly around a conductor an E.M.F is used inside the conductor which produces electric current.
Electromagnetic induction is the basic principle of electric generator.
at some point the velocity of the ball will reach 0 so we can say that Vf=0
then we have to divide the total time by 2 for upward and down ward motion of the ball which gives us 8.25s/2=4.125s
now the given are
for the formula of initial velocity