Using the moment the porpoise re-enters the water as the origin, and the downward direction to be positive, it will reach a depth y after time t of
y = (8.9 m/s) t + 1/2 (- 9.3 m/s²) t²
with velocity v of
v = 8.9 m/s + (- 9.3 m/s²) t
Find the time it takes for the porpoise to come to a stop:
0 = 8.9 m/s + (- 9.3 m/s²) t
t = (8.9 m/s) / (9.3 m/s²)
t ≈ 0.96 s
After such time, its depth is
y = (8.9 m/s) (0.96 s) + 1/2 (- 9.3 m/s²) (0.96 s)²
y ≈ 4.3 m
