If an object is dropped from a tall building and hits the ground 3.0 s later, how tall is the building?

Ignoring air resistance . . .

-- Gravity adds 9.8 m/s to the speed of a falling object every second.

-- After falling for 3 sec, it's falling (3 x 9.8) = 29.4 m/s faster
   than it was when it was dropped.

-- It started out with zero speed when it rolled off the top of the building,
   so its speed is  29.4 m/s after 3 seconds.

-- Its average speed all the way down is

               (1/2) (0 + 29.4)  =  14.7 m/s .

-- After traveling for 3 sec at an average speed of  14.7 m/s,
   it has traveled

                   (3 sec) x (14.7 m/s)  =  44.1 meters .

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