Question 185707
If an object is given an initial velocity of v0 feet per second from a height of S0 feet, then its height S after t seconds is given by the formula 
S = -16t^2 +v0t + S0. A rock is thrown into the air at 26 feet per second from a height of 13 feet. How long does it take to reach the earth?
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S(t) = -16t^2 +26t + 13
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When it reaches the earth its height is zero; solve for "t":
-16t^2 +26t + 13 = 0
 
t = [-26 +- sqrt(26^2 - 4*-16*13)/(-32)

t = [-26 +- sqrt(1508)]/(-32)

Positive solution:

t = [-26 - 38.83]/(-32)

t = 2.03 seconds
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Cheers,
Stan H.