A rock is tied to string and swung in a horizontal circle around your head. The velocity is doubled while halving the length of the string. A new rock that is twice the mass as the original rock is now attached to the string. The more massive rock is swung around by the same centripetal force as before. By what factor did the time to go around change by?
This YouTube video can be found at http://youtu.be/hDIo3Uhfl6A