SOLUTION: Can someone please help me out, howevever i am not good in algebra at all, especially word problems. A ball is thrown vertically upward from the ground. |Its distance in feet f

Algebra ->  Equations -> SOLUTION: Can someone please help me out, howevever i am not good in algebra at all, especially word problems. A ball is thrown vertically upward from the ground. |Its distance in feet f      Log On


   

Question 417597: Can someone please help me out, howevever i am not good in algebra at all, especially word problems.
A ball is thrown vertically upward from the ground. |Its distance in feet from the ground in t seconds is s= -16t to the second power +96t. After how many seconds will the ball be 128 feet from the ground?
_________ seconds
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
d+=+-16t%5E2+%2B+96t
It will help to think of the equation this way:
Gravity acts in a downward direction, so the term -16t%5E2
is the only thing that can represent gravity.
The other term 96t is the only thing that can represent
the distance caused by the upward throwing force.
128+=+-16t%5E2+%2B+96t
Divide both sides by 16
-t%5E2+%2B+6t+=+8
-t%5E2+%2B+6t+-+8+=+0
Use quadratic formula
t+=+%28-b+%2B-+sqrt%28+b%5E2-4%2Aa%2Ac+%29%29%2F%282%2Aa%29+
a+=+-1
b+=+6
c+=+-8
t+=+%28-6+%2B-+sqrt%28+6%5E2-4%2A%28-1%29%2A%28-8%29+%29%29%2F%282%2A%28-1%29%29+
t+=+%28-6+%2B-+sqrt%28+36+-+32+%29%29%2F%28-2%29+
t+=+%28+-6+-+2+%29+%2F+%28-2%29+
t+=+4+ sec (the other root gives a negative for t)