[填空题]
A marble of mass m and radius r rolls alongwbe h74v0r90cbl1 w gg the looped rough trackctv*o, e 3d;vyb pix)5 :ps8coa wxs.0 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 track? Assubao)cx.c3;ovi dy8t0*s, xp ps5:w ve me $r\ll R$ and ignore frictional losses. h = R