A small rock sits on the edge of a tall building. A strong wind blows the rock off the edge. The distance, in feet, between the rock and the ground tseconds after the rock leaves the edge is given by d=−16t^2−4t+472.
How many seconds after the rock leaves the edge is it 460 feet from the ground? How many seconds after the rock leaves the edge does it hit the ground? Please help!

Question

A small rock sits on the edge of a tall building. A strong wind blows the rock off the edge. The distance, in feet, between the rock and the ground tseconds after the rock leaves the edge is given by d=−16t^2−4t+472.
How many seconds after the rock leaves the edge is it 460 feet from the ground? How many seconds after the rock leaves the edge does it hit the ground? Please help!

anonymous · Answer

i'll fan and give a medal!

geerky42 · Answer

So $d$ is distance between rock and ground.

"How many seconds after the rock leaves the edge is it 460 feet from the ground?"

So $d=460$, so subtract both sides by 460 then apply quadratic formula. Just remember that t has to be positive.

"How many seconds after the rock leaves the edge does it hit the ground? "

This time, $d=0$

So just apply quadratic formula right there. Again, t has to be positive.

anonymous · Answer

do you know the answers? i don't know what i'm doing wrong..

anonymous · Answer

@geerky42  for "How many seconds after the rock leaves the edge is it 460 feet from the ground?" I am getting 313.94

anonymous · Answer

Nevermind, I guess i put it in my calculator wrong.. I ended up getting .75 for the first answer. Now I am working on the second answer.

geerky42 · Answer

You know quadratic formula?

$t = \dfrac{-b\pm\sqrt{b^2-4ac}}{2a}$, from $0 = at^2+bt+c$

geerky42 · Answer

Yeah I got that as well.

anonymous · Answer

How should I be putting this into my calculator? Everytime I try I get the wrong answer.

anonymous · Answer

@geerky42