[填空题]
A marble of mass m and radius r rolls along the looped rough track of Fipm-g klvy*p:ffb+ 8i7mbf,z33;cvsf69xq dv1 h ry,d g. 8–58. What is the minimum valdqmx z9 yf3f,3c;6 ,dhv r1sbvue of the vertical height h that the marble must drop if it is to reach the highest point of the loop without leaving the track? Assume $r\ll R$ and ignore frictional losses. h = R