Influence Line Diagram for Bending Moment at P:
Next, we shall draw the influence line diagram for bending moment (MP) at a point P which is at a distance 'a' from end A :
While the load is among A and P the BM at P is
MP = - 1 × (a - x) = - (a - x)
Hence, the ordinate of the IL diagram is - a when x = 0 (at point A) and it is 0 when x = a (at point P).
As soon as the load crosses over P to the right hand side the BM at P = 0 and remains as such till the end.