[填空题]
A marble of mass m a6r+mxx ;uivzkc*q 4. hnd rapmce(8ges ,26h-utk k+mi9v wdius r rolls along the looped rough track of Fig. 8–58. What is the minimum value of the vertical height hm-t k s,(wm62gceu9ihv+e p k8 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