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

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.