SOLUTION: Could someone please help me solve this word problem I am not understanding how to solve it. A ball is thrown upward from the roof of a building 100 m tall with an initial veloc

Question 101576: Could someone please help me solve this word problem I am not understanding how to solve it.
A ball is thrown upward from the roof of a building 100 m tall with an initial velocity of 20 m/s. When will the ball reach a height of 80 m?

Answer by ankor@dixie-net.com(22740) (Show Source): You can put this solution on YOUR website!
A ball is thrown upward from the roof of a building 100 m tall with an initial velocity of 20 m/s. When will the ball reach a height of 80 m?
:
This can be made into a quadratic equation with the three elements
Let t = time in seconds
1. Gravity (negative because it's accelerated downward) -4.9t^2
2. Initial velocity (positive because it's thrown up-ward: + 20t
3. Initial height above ground: +100
:
-4.9t^2 + 20t + 100 = height above ground
:
They want to know when will it be at a height of 80 ft:
-4.9t^2 + 20t + 100 = 80
:
-4.9t^2 + 20t + 100 - 80 = 0; subtracted 80 from both sides
:
-4.9t^2 + 20t + 20 = 0
:
This is the quadratic formula: except we will be solving for t instead of x:

Substitute: a=-4.9; b=20; c=20

Do the math:

; minus a minus is a plus
:

:
; take the square root of 792
Solution 1

t = -.83, this is NOT our solution
:
Solution 2:

; neg into a neg = +
;
t = + 4.9 sec, this IS our solution, (Has to be positive)
:

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