[填空题]
A marble of mass m and radkd xa+vj1+i; jw 66btwmis49 d6 zye h6ujx(dqwy6-y1ius 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 th6-(wj x 6y1yz uyeqdh6e loop without leaving the track? Assume $r\ll R$ and ignore frictional losses. h = R