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

Algebra ->  Quadratic Equations and Parabolas  -> Quadratic Equations Lessons  -> Quadratic Equation Lesson -> SOLUTION: A ball is thrown upward from the roof of a building 100 m tall with an initial velocityof 20 m/s. When will the ball reach a height of 80 m?      Log On


   

Question 95446: A ball is thrown upward from the roof of a building 100 m tall with an initial velocityof 20 m/s. When will the ball reach a height of 80 m?
Answer by scott8148(6628) About Me  (Show Source):
You can put this solution on YOUR website!
h=(-g/2)t^2+vt+s ... where g is gravitational acceleration (9.8 in meters), v is initial velocity, and s is starting height

80=-4.9t^2+20t+100 ... 0=-4.9t^2+20t+20 ... using quadratic equation gives

t=%28-20+%2B-+sqrt%2820%5E2-4%2A%28-4.9%29%2A20%29%29%2F%282%28-4.9%29%29 ... t=-0.83 and t=4.91 ... the negative value is not realistic