关于门的力矩的物理题（要详细解释）
已知一个门的高度为2.1m（在y轴上）,宽度为0.8m（在x轴上）,hinge（铰链）一个从门最上端下来0.3m长,另一个从门最下端上去也是0.3m长,每个都支撑一半的门的重量（一半的门重量是30kg）,问上端的铰链对门的力和下端的铰链对门的力.要用矢量表示（就是在y轴上和x轴）
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?
这是原题
铰链的坐标为什么不用铰链的重心呢？
一楼的既然方法没错，那（Ny+Ty)×0.4+（Nx+Tx)×0.75=0
Nx+Tx=0，这怎么回事？