Question

What is the average velocity of a dropped brick that falls 4.0 m?

vi = 4.0 m/s
vf = 0 m/s
a = 9.81 m/s^2

t = vf-vi / a
t = -4/9.81
t = 0.41 s

v = d/t
v = 4.0/0.41
v = 9.81 m/s^2 <----------

Answer 1

actually I think vi, your initial speed is zero
we do not know vf, the final speed. All we know is that it fell 4 m.

h = Hi + vi t - (1/2) g t^2
0 = 4 + 0 - (1/2)(9.81) t^2
so
t^2 = 4/4.9
t = .9035 seconds to fall 4 meters

the average speed is 4 meters/.9035seconds
or 4.43 m/s

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by the way you can find vf, the final speed when it hits the ground
vf = 9.81 t = 9.81(.9035) = 8.86 m/s
the average is then (0 +8.86)/2
which is sure enough 4.43 m/s