You are walking on a bridge and accidentally drop your cell phone. if the bridge is 45.4 m above the water, how long does it take the phone to hit the water?

The phone is in free fall, so it is moving by uniformly accelerated motion, with constant acceleration equal to  (gravitational acceleration). Taking the water as reference point, the vertical position of the phone at the istant t is

where h=45.4 m is the height of the bridge. The phone hits the water when y(t)=0, so by requiring this condition we find the time t after which the phone hits the water:

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