physics
posted by Mel .
An object is thrown straight down off of a cliff with a velocity of 18m/s.
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?

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.