You drop a rock down a well that is 11.5 m deep. how long does it take the rock to hit the bottom of the well ?

The acceleration of gravity depends on the distance from
the center of the Earth.  We use 9.8 m/s² for any place that's
on the Earth's surface, or reasonably close.  The 11.5 meters
down into the well certainly qualifies.

The formula we want is: 

                          Distance of fall = (1/2) · (acceleration) · (time)²

                                  11.5 m      = (1/2) (9.8 m/s²) (time)²

Divide each side
by  4.9 m/s² :            (11.5m) / (4.9 m/s²)  =  time²

                                         2.347 sec²        =  time²

                                   time = √(2.347 sec²)  =   1.53 seconds  .

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