SOLUTION: A ball is thrown upward from the roof of a 120-ft building with an initial velocity of 20 ft/s. How long does it take the ball to fall back to the ground?

Question 112161: A ball is thrown upward from the roof of a 120-ft building with an initial velocity of 20 ft/s. How long does it take the ball to fall back to the ground?
Answer by Earlsdon(6294) (Show Source): You can put this solution on YOUR website!
The height of an object propelled upwards from an initial height of with an initial velocity of is given by the function (height as a function of time):

In your problem:
ft/sec. and
ft.
You want to find out at what time does the height (h) = 0, so, making the appropriate substitutions into the equation, we get:

Using the quadratic formula to solve:
Simplifying this, we get:

or
or
Only the positive value is meaningful here, so the ball reaches the ground in 3.434 seconds.

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