Respuesta :

sqdancefan
The equation of motion can be
  h(t) = -16t^2 + 4
for a penny dropped from a height of 4 feet. Then h(t) = 0 for
  0 = -16t^2 +4
  t^2 = 4/16
  t = √(1/4) = 1/2

It takes about 1/2 second for the penny to hit the floor.