word problem

Question

a ball is thrown upward with an initial velocity of 64 ft/sec from a height of 720ft. its height s, in feet, after t secondes is given by s = -16t ^2 +64t+720.after how long will the ball reach the ground?

 

Answer 1

Set s = 0 and solve for t.

 

 -16t² + 64t + 720 = 0

 

Divide both sides by -4

 

t² - 4t - 45 = 0

 

Factor

 

(t - 9)(t + 5) = 0

 

t = {-5,9}

 

We take the positive value of 9 seconds to determine that it takes 9 seconds to hit the ground. We ignore t = -5 because tells us how far into the past the ball had been thrown if it had been thrown from the ground upwards and produced the same trajectory.