Question 1153146
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(a)  To determine "how long . . . ", solve the equation

         16*t^2 = 400.


     It is easy to do

         t^2 = {{{400/16}}} 

         t^2 = 25

         t = sqrt(25) = 5 seconds.    <U>ANSWER</U>



(b)  The average velocity = {{{total_distance/total_time}}} = {{{400/5}}} = 80 ft/s.      <U>ANSWER</U>



(c)  In Calculus,  velocity is first derivative of the distance over the time


         v = {{{(ds)/(dt)}}} = {{{(d/(dt)) (16t^2)}}} = 2*16*5 = 160 ft/s.


     In Physics,  v = g*t, where g is the gravity acceleration g = 2*16 ft/s^2 = 32 ft/s^2,  t = 5 seconds, 

     so  v = 32*5 = 160 ft/s,  giving the same <U>ANSWER</U>
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Solved, answered, explained and completed.