[填空题]
A marble of mass m and radiu 0 pdjxuwc)dw*d)b:h;7uu*f x,be0dvs 7.qguj4x t2x s 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 drox, x0 st7edb4qvujg2 .p 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
