Hello, I have a question about Lecture#7. At around 42:23 (time in the lecture clip), Professor Lewin calculates the maximum height to be 900meters. I also tried to calculate this by setting the y-component of velocity to zero, but I got a different answer. Can someone please explain how to get the 900meters and 30 seconds answer? Thanks in advance!
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-7/

Question

Hello, I have a question about Lecture#7.  At around 42:23 (time in the lecture clip), Professor Lewin calculates the maximum height to be 900meters.  I also tried to calculate this by setting the y-component of velocity to zero, but I got a different answer.  Can someone please explain how to get the 900meters and 30 seconds answer?  Thanks in advance!
http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-7/