shear stress
1. gradient (steeper = gravity more effective).2. slope undercutting (natural v human).
Shear stress is the force that a fluid or solid exerts on a surface parallel to the surface. It is the result of an applied force acting parallel to the surface of an object. When a force is applied to an object in a direction that is parallel to the surface of the object, shear stress occurs. For instance, when one layer of fluid moves horizontally past another layer, shear stress occurs between the two layers, creating a frictional force that opposes the motion. Shear stress can also occur in solids, for instance, when two separate parts of a material move past each other at different speeds causing friction. The unit of shear stress is Newton per square meter (N/m2) in SI units, and pound-force per square inch (psi) in the Imperial system. Shear stress is a critical factor in mechanical engineering as it can cause fatigue and failure of materials under excessive loads.
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