SOLUTION: Develop a height equation: A ball thrown upward from the roof of a building 100m tall with an initial velocity of 20m/s. When will the ball reach a height of 80m?

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Question 88620: Develop a height equation: A ball thrown upward from the roof of a building 100m tall with an initial velocity of 20m/s. When will the ball reach a height of 80m?
Answer by Nate(3500) About Me  (Show Source):
You can put this solution on YOUR website!
f%28x%29+=+%28-1%2F2%29gx%5E2+%2B+vx+%2B+h
f(x) determines height after x seconds
g is the gravitational constant
v is the initial velocity
h is the initial height
80+=+%28-1%2F2%2932x%5E2+%2B+20x+%2B+100
-20+=+-16x%5E2+%2B+20x
5%2F4+=+x%5E2+-+5x%2F4%29
105%2F64+=+80%2F64+%2B+25%2F64+=+5%2F4+%2B+25%2F64+=+%28x+-+5%2F8%29%5E2
0+%2B-+sqrt%28105%29%2F8+=+x+-+5%2F8
%285+%2B-+sqrt%28105%29%29%2F8+=+x
After %285+%2B+sqrt%28105%29%29%2F8 seconds, the ball would be 80m high.
graph%28300%2C400%2C-3%2C4%2C-20%2C120%2C80%2C-16x%5E2+%2B+20x+%2B+100%29