-- Gravity makes a falling object fall 9.8 m/s faster every second.
-- So, it reaches the speed of 30 m/s in (30/9.8) = 3.06 seconds after it's dropped.
-- The distance an object falls from rest is D = 1/2 (acceleration) (time)²
D = 1/2 (9.8 m/s²) (3.06 sec)²
D = (4.9 m/s²) (9.37 sec²)
D = 45.8 meters
Notice that we don't care how high the building is. The problem works just as long as the object can reach 30 m/s before it hits the ground. That turns out to be anything higher than 45.8 meters for the drop . . . maybe something like 13 floors or more.
Now I'll go a little farther for you ! Writing the last paragraph made me a little curious and uncomfortable. So I went and looked up the world's tallest buildings . . . and I found out that this problem could never happen !
The tallest building in the world now is the Burj Khalifa, in Dubai. It has 163 floors, and it's 828 meters high ! That's 2,717 feet. It's gonna be a long time before there's a building that's 1125 meters tall, like this problem says. That's close to 3700 feet . . . I've had flying lessons where I wasn't that far off the ground !
