Why does a cylindrical bottle/jar keep rolling back and forth once given an initial push, like a SHM? It is really a SHM? If so, what force becomes the restoring force (F=-kx) ? Please watch this video: [url=http://youtu.be/nosZ2p07YJo]http://youtu.be/nosZ2p07YJo[/url]

http://www.youtube.com/watch?v=nosZ2p07YJo

The motion of the jar in the video is not very smooth, probably due to its irregular surface. If the jar or bottle is perfectly cylindrical, and the floor is also perfectly horizontal, the motion should be smooth. Similar motion is observed when the jar/bottle is partially/fully filled with water.

Could someone explain such a motion?