Shear: An system of internal forces whose resultant is a force acting perpendicular to the longitudinal axis of a structural member or assembly: sometimes called shear force.

Shear stress: Stress resulting when two forces act on a body in opposite directions in parallel adjacent planes

To put it simply, shear force is the force applied on the same body in opposing directions. When this force overcomes shear strength, the object is either fractured, or split apart.

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