Question

A firecracker shoots up from a hill 145 feet high with an initial speed of 80 feet per second. Using the formula H(t) = -16t2 + vt + s, approximately how long will it take the firecracker to hit the ground?
a. 13 seconds b. 11 seconds c. 9 seconds d. 6 seconds

Answer 1

145 = -16 t² + 80 t +s
s = ?
v = v0 - at
0 = 80 - at
80 = at
s = 80 t - at²/2
s = 80 t - 80t/2
s = 40 t
Finally: 145 = -16 t² + 80 t + 40 t
16 t² - 120 t + 145 = 0
We will solve this quadratic equation:
t = [120 +√ ( (-120)²- 4 · 16 · 145 )] / 32
t = [120 + √ (14400 - 9280) ] / 32 
t = (120 + 71.55) / 32 = 5.98 ≈ 6 s
Answer:D) 6 seconds