Question

Neglecting the effect of air resistance a stone dropped off a 175-m high building lands on the ground in: A)3s B)4s C)6s D)18s E)36s

Answer 1

Answer:

c) 6s

Explanation: take downwards as positive.

let vf = the velocity of the stone when it hits the ground, vi = 0  be the starting velocity of the stone. r0 = 175m be the height of the building.

(vf)^2 = (vi)^2 + 2g(r-r0)

(vf)^2 = 2g(r0)

     vf =  \sqrt{2g(r0)}  

         = \sqrt{2×9.8×(175)}  

         = 58.6 m/s

then:

vf = vi + g×t

vf = g×t

 t = vf/g

   = (58.6)/(9.8)

   = 6s

Therefore, it will take the stone 6 seconds to reach the gound.