A firecracker shoots up from a hill 160 feet high with an initial speed of 105 feet per second. Using the formula H(t) = −16t2 + vt + s, determine how long it will take the firecracker to hit the ground.

7.5 seconds

7.8 seconds

8.1 seconds

8.3 seconds

Question

A firecracker shoots up from a hill 160 feet high with an initial speed of 105 feet per second. Using the formula H(t) = −16t2 + vt + s, determine how long it will take the firecracker to hit the ground.

 7.5 seconds

 7.8 seconds

 8.1 seconds

 8.3 seconds

anonymous · Answer

$$ H(t) = −16t^2 + vt + s$$ you are told $v=105,x=160$ so this becomes 
$$H(t)=-16t^2+105t+160$$
when it hits the ground $H=0$ so your actual job is to solve 
$$-16t^2+105t+160=0$$ for $t$

anonymous · Answer

you can use the quadratic formula i guess, it is going to be ugly and you will need a calculator

anonymous · Answer

wow it is going to be really really really ugly. i would use wolfram

shane_b · Answer

Yep :) http://www.wolframalpha.com/input/?i=solve+for+t%3A+0%3D-16t^2%2B105t%2B160

anonymous · Answer

so then its the second option?