Question

A stone is thrown straight downward with a speed of 20 m/s from the top of a tall building. If the stone strikes the ground 3.0 seconds later, about how tall is the building?

A) 45m
B) 60m (wrong)
C) 90m
D) 105 m

Answer 1

Do you remember this formula for the distance traveled while accelerated ?

Distance = (initial speed) x (t) plus (1/2) x (acceleration) x (t²)

I think this is exactly what we need for this problem.

initial speed = 20 m/s down
acceleration = 9.81 m/s² down
t = 3.0 seconds

Distance down = (20) x (3) plus (1/2) x (9.81) x (3)²

Distance = (60) plus (4.905) x (9)

Distance = (60) plus (44.145) = 104.145 meters

Choice D) is the closest one.