a mass 3 kg is dropped from a 20m high stand how long does it take to reach the ground?
neglecting air friction it does not matter what the mass might be
a = -9.8
h = Ho + Vi t - (1/2)(9.8) t^2
0 = 20 + 0 - 4.9 t^2
t = sqrt (20/4.9) about 2 seconds