what is shear force

Shear force and bending moment are examples of interanl forces that are induced in a structure when loads are applied to that structure. Loading tends to cause failure in two main ways:
a) by shearing the beam across its cross-section.
b) by bending the beam to an excessive amount.
Shear force may be defined as "the algebraic sum of the loads to the left or right of a point (such that the addition of this force restores vertical equilbrium)".
The accepted sign convention is:
+ve shear
sum of forces to the left is upwards (+ve)
sum of forces to the right is downwards (-ve)
-ve shear
sum of forces to the left is downwards (-ve)
sum of forces to the right is upwards (+ve)
A shear force diagram is one which shows variation in shear force along the length of the beam.
Bending moment may be defined as "the sum of moments about that section of all external forces acting to one side of that section"