關(guān)于門的力矩的物理題（要詳細(xì)解釋）
已知一個門的高度為2.1m（在y軸上）,寬度為0.8m（在x軸上）,hinge（鉸鏈）一個從門最上端下來0.3m長,另一個從門最下端上去也是0.3m長,每個都支撐一半的門的重量（一半的門重量是30kg）,問上端的鉸鏈對門的力和下端的鉸鏈對門的力.要用矢量表示（就是在y軸上和x軸）
A door has a height of 2.1 m along a y axis that extends vertically upward and a width of 0.80 m along an x axis that extends outward from the hinged edge of the door.A hinge 0.30 m from the top a hinge 0.30 m from the bottom each support half the door's mass which is 30 kg.
(a) What is the force on the door at the top hinge?
(b) What is the force on the door at the bottom hinge?
這是原題
鉸鏈的坐標(biāo)為什么不用鉸鏈的重心呢？
一樓的既然方法沒錯，那（Ny+Ty)×0.4+（Nx+Tx)×0.75=0
Nx+Tx=0，這怎么回事？