Auto-syncing JUST the music on an iPod

I use my iPod for music as well as movies. I also keep my music library on my MacBook, but to save hard drive space I do not keep the movies on my computer, only my iPod. I tried forever to make iTunes sync just my music but no matter what I did it would always erase the movies on my ipod even when I told iTunes not to sync movies. Does anyone know how I could sync my music with my computer without it erasing my movies from my iPod?

I know this sounds stupid, "just check sync music and uncheck sync movies dummy" but I have tried such things and it doesn't work. Is this impossible to do because of stupid copyright laws or what?