When you render and export a movie, the default ouptut settings are for files that are not intended for playback, see this answer from Adobe:
FAQ: Why is my output file huge, and why doesn't it play back smoothly in a...
Johnny Cuevas, Editor
Thinkck.com
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.