Since raindrops grow as they fall, their surface area increases and therefore the resistance to their falling increases. A raindrop has an initial downward velocity of 10 m/s and its downward acceleration is given by the function below. If the raindrop is initially 500 m above the ground, how long does it take to fall? (Give your answer correct to two decimal places.)
a={(9-0.9t ( if ) 0<=t<=10,
0 ( if ) t > 10)
____ s