An elevator is descending with uniform acceleration. In order to measure the acceleration, a person in the elevator drops a stone just when the elevator starts. The stone is 6 feet above the floor of the elevator when it is dropped. It is seen by the person that the stone strikes the floor of the elevator in 1 second. What can you say about the acceleration of the elevator ?

$20 ft/s^2$

Acceleration of the elevator : $a$

Acceleration of the stone w.r.t. the ground : $g$

Acceleration of the stone w.t.t. the elevator : $a_{stone/elevator}$

$a_{stone/elevator} = g - a$

  = $9.8 m/s^2 - a$

  = $32 ft/s^2 - a \text{ ... [9.8 m/s^2 = 32 ft/s^2]} $

Displacement : s = 6 ft

Time taken : t = 1 

Initial velocity : u = 0

$\text{ Using} : S = ut + \frac{1}{2} at^2$

$6 = 0*1 + \frac{1}{2}(32-a)*1^2$

$6 = \frac{1}{2}(32-a)$

$a = 20 ft/s^2$