There is a well-known legend about physicist Isaac Newton. It says that he was inspired into his development of the Law of Gravitation while resting under a tree and having an apple fall (free-fall) on his head from one of the branches of the tree. Suppose the apple was 3.90 meters above ground before falling, while Newton's head was 90 centimeters above ground. How long did it take the apple to hit Newton after falling off?