a smart phone

Question

A smart-phone is thrown upwards from the top of a 480-foot building with an initial velocity of 16 feet per second. The height h of the smart-phone after t seconds is given by the quadratic equation h= -16t²+16t+480 . When will the smart-phone hit the ground?

Answer 1

When h = 0. First, I'm going to divide by -16.

 

t² - t - 30 = 0

(t - 6)(t + 5) = 0

 

t - 6 = 0

t = 6 seconds

 

t + 5 = 0

t = -5

Well, we can't go back in time, so we disregard this answer. This is where the left side of the parabola would touch the x-axis if we graphed the full parabola, but the y-intercept is the top of the building.