Question

If you stand at the edge of a cliff that is 75 m high and throw a rock directly up into the air with a velocity of 20 m/s, at what time w ill the rock hit the ground?

Answer 1

Rock will hit the ground after a time of 6.45 s

initial velocity=20 m/s

initial displacement=Yo=75m

final displacement=0

using the kinematic equation

Y= Yo+ V t+ 1/2 gt²

0=75+20 t+1/2 (-9.8)t²

-4.9t²+20 t+75=0

This is a quadratic equation in t,s we use the quadratic formula

t=$\frac{-20+-\sqrt{20^{2}-4(-4.9)(75)}}{2(-4.9)}$

t=6.45 s