What is shear force?

• A. A force acting parallel to the surface
• B. A force acting perpendicular to the surface
• C. A force rotating about an axis
• D. A force pulling away from the surface

Answer: (A)

Shear force is a force that acts parallel to a surface, causing adjacent parts of a material to slide past one another. This is different from a force that acts perpendicular to the surface (normal force, which pushes straight in or out) and from a force that causes rotation around an axis (torque). It’s also distinct from a force that pulls away from the surface (tension). A simple example is pushing sideways on a stack of cards so the layers slide over each other—the force is parallel to the surface. That parallel action is what defines shear force.