Question

A ball is dropped from the top of a 68 m building. How long does it take for the ball to hit the ground?

In the previous problem, what is the speed of the object the instant before it hits the ground?

Answer 1

consider the motion of the ball in vertical direction

v_o = initial velocity = 0 m/s   

a = acceleration = acceleration due to gravity = 9.8 m/s²

y = displacement of the ball from top of building to the ground = height of the building = 68 m

t = time taken to travel

using the kinematics equation

y = v_o t + (0.5) a t²

68 = 0 t + (0.5) (9.8) t²

t = 3.73 sec

v = final velocity just before hitting the ground

using the equation

v = v_o + a t

v = 0 + (9.8) (3.73)

v = 36.6 m/s