Answer:
V = a * t for uniformly accelerated motion starting at zero speed
V = 4 s * 9.8 m/s^2 = 39.2 m/s
S = V0 t + 1/2 a t^2 = 1/2 a t^2
S = 1/2 * 9.8 m/s^2 * 16 s^2 = 78.4 m
Check:
Average speed = 39.2 / 2 = 19.6 m/s
S = 4 * 19.6 = 78.4 m
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