[quote author=tanghb link=topic=2324.msg8709#msg8709 date=1320474764]
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]

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?
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I hope the following simulation might help to solve it.

/htdocs/ntnujava/ejsuser/2/users/ntnu/fkh/rollingBottle_pkg/rollingBottle.propertiesFull screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(1028.2kB):double click downloaded file to run it. (5 times by 4 users) , Download EJS source View EJS source