Force by ground on the cannon = 100N.
Work done by ground on the cannon = 100 x(0.5) = 50J.
Applying conservation of energy
work done by ground on canon = Kinetic energy of canon immediately after firing = (1/2) mv2= 50J.
Velocity of cannon (v) = 0.5m/sec.
By applying law of conservation of momentum
400(0.5) = 2 (velocity of cannon ball)
Velocity of canon ball = 100m/sec.
Time taken by the canon ball to hit = 500/100 = 5 seconds.