A firecracker shoots up from a hill 165 feet high with an initial speed of 110 feet per second. Using the formula H(t) = −16t^2 + vt + s, determine how long it will take the firecracker to hit the ground.

Question

nathan917 · Answer

We have v = 105 ft/s and s = 160 ft. Sub those in:

H(t) = -16t^2 + 105t + 160

We want to know when this hits the ground, so set H(t) equal to zero:

0 = -16t^2 + 105t + 160

Use the quadratic formula:

t = [1 / (2 * -16)][-105 ± √((105)^2 - 4(-16)(160))]
t = (105 / 32) ± (1 / 32)√(21265)

So we have two roots, but we obviously want the positive one, because negative time makes no sense. Thus:

t = (105 / 32) + (1 / 32)√(21265) s
t ≈ 7.84 s 

The answer is B.

anonymous · Answer

howd u know it was b? lol
u taking conections?

nathan917 · Answer

Yes. but I'm Not In Your grade :P


7.5 seconds

7.8 seconds

8.1 seconds

8.3 seconds


These you options? :P

anonymous · Answer

it wasnt in that oreder but 7.8 was there so i got it it was c. on mine lol

nathan917 · Answer

:P. They change it every year or so.

anonymous · Answer

oh lol ... well thnx :)