An object is thrown straight down off of a cliff with a velocity of 18m/s.

Question

a) What is the object's velocity after 15s?
b) If the cliff is 358m high, how long does it take for the object to hit the ground?

Answer 1

Assuming acceleration is the force of gravity.

Vf = Vi + a*t
Vf = 18m/s + 9.81ms^-2 * 15s
Vf = 165.15 m/s

d = Vi*t + 1/2a*t^2
358 = 18m's*t + (1/2)9.81ms^-2 * t^2
Set it to zero:
0 = 4.91ms^-2 * t^2 + 18m/s * t - 358.
Use quadratic formula

t = 6.9s or t = -10.5s

Can't have negative time. Therefore it takes 6.9 seconds.