bending moment 是 shear force 和 distance 的product
it is simply the intregration of shear force vs distance graph.
or usually we define it as area below shear force vs distance graph
程式因support condition 和 load pattern而定的.
只有在meet到一定的support condition和load pattern才能用。
如果没meet到condition你就要用会我刚说的shear force vs. distance的方法了