設(shè)物體經(jīng)過(guò)O點(diǎn)正上方時(shí)對(duì)支架的壓力大小為N．以O(shè)為支點(diǎn)．
當(dāng)支架和小物體間光滑，小物體經(jīng)過(guò)O點(diǎn)正上方時(shí)，根據(jù)力矩平衡得知N₁的力矩與物塊對(duì)支架壓力的力矩平衡，即：
M_N1=M_N．
當(dāng)小物體勻速下滑經(jīng)過(guò)O點(diǎn)正上方時(shí)，N₂的力矩與摩擦力的力矩之和等于物塊對(duì)支架壓力的力矩，即M_N2+M_f=M_N．
由于兩次物塊對(duì)支架壓力的力矩相等，得M_N1=M_N2+M_f，則有M_N1＞M_N2，而力臂不變，所以N₁＞N₂．
故選C