SOLUTION: An object is propelled vertically upward with an initial velocity of 20 m/s. The distance s (in meters) of the object from the ground after t seconds is given by {{{ s(t) = -4.9t^2

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Question 1120524: An object is propelled vertically upward with an initial velocity of 20 m/s. The distance s (in meters) of the object from the ground after t seconds is given by . When will the object be 15 meters above the ground?
Found 2 solutions by Alan3354, ikleyn:
Answer by Alan3354(69443)   (Show Source): You can put this solution on YOUR website!
. When will the object be 15 meters above the ground?
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when s(t) = 20


Solve for t.
You get 2 times, ascending & descending.
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Ooooh, 15, not 20.
The method is the same.
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If you solve for s(t) = 100, eg, you don't get a real number solution because the projectile never gets to 100 meters.
Answer by ikleyn(52751)   (Show Source): You can put this solution on YOUR website!
.
= . When will the object be 15 meters above the ground?
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I am here to edit the Alan's solution, which is not exactly correct. 

when s(t) = 15.


 = 




Solve for t.


You get 2 times, ascending & descending.







Plot y =  (red)  and  y = 15 (green)


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