An object is propelled vertically upward from the top of a 256-foot building. The quadratic function s(t)=-16t^2+144t+256 models the ball's height above the ground, s(t) in feet, t seconds after it was thrown. How many seconds does it take until the object finally hits the ground? Round to the nearest tenth of a second if necessary.

a. 1.5 seconds
b. 4.5 seconds
c. 10.5 seconds
d. 2 seconds

Question

An object is propelled vertically upward from the top of a 256-foot building. The quadratic function s(t)=-16t^2+144t+256 models the ball's height above the ground, s(t) in feet, t seconds after it was thrown. How many seconds does it take until the object finally hits the ground? Round to the nearest tenth of a second if necessary.

a. 1.5 seconds 	
	b. 4.5 seconds 	
	c. 10.5 seconds 	
	d. 2 seconds

alfie · Answer

Hitting the ground => final Y position = 0
sooo...

anonymous · Answer

when the object is on the ground the hight is zero. therefore s(t)=0 therefore 16t^2+144t+256=0 
t^2+9t+16=0 (dividing by 16)
using quadratic formula (-9+-root(9^2-(4x-1x16))/2=t
t=(-9+-(root (145))/2 = either 1.52... or -10.5.... it can't be the negative one therefore answer a

hero · Answer

Answer is C

alfie · Answer

Yup, it's C. Gabriel you messed up a minus sign in front of the sixteen back there :). But good job on getting the problem straight ;)

hero · Answer

Oh well, too late now

alfie · Answer

too late for what? o_O

hero · Answer

The OP is gone

alfie · Answer

What does OP stand for? lol (i phail)

hero · Answer

Original Poster

alfie · Answer

oh well, :)... ! The hope is he got how to do it, and not just how to copy a quadratic equation solution :D...!

anonymous · Answer

just didn't divide through by minus 16 was all, i only noticed it half way through answering it because of my bad eyesight :p

anonymous · Answer

i'm here, had to step away for a minute.. so... it's c?

anonymous · Answer

yep seems so.