[填空题]
A marble of mass m and rlw 11y /ac,xfok1bj mxc( ah1z-ci09 qa d ;(nesj631 nr:zesjcdius r rolls along the looped rough track of Fig. 8–58. What is the minimum value of the vertical height h that the marble must drop if it is to reach the highest point of the loop without leaving the tr6 d cn:s 1js(r3;ejnezack? Assume $r\ll R$ and ignore frictional losses. h = R