Expert: Steve Holleran - 9/9/2008

Question
A ball was thrown up from the ground. Its distance in feet from the ground in t seconds is
S=-16t squared + 128t after how many econds will the ball be 240 feet from the ground

Answer
Hi Stephani,

Okay, so we've got a height function

                     s = -16t^2 + 128t

and we want the height to be 240, so let's substitute this for s:

                   240 = -16t^2 + 128t

                   16t^2 - 128t + 240 = 0

divide by 16:         t^2 - 8t + 15 = 0

                     (t - 5)(t - 3) = 0   so t = 3, 5.

Remember, the t=3 is on the way up and the t = 5 on the way down.

Hope this helps
Steve