Shear stress is a measure of how much force is applied parallel to a surface area. It occurs when layers of material slide past each other, which can happen in solids, liquids, and gases. The formula for shear stress is the force divided by the area over which it acts, often expressed in units like pascals (Pa). In engineering and physics, understanding shear stress is crucial for designing structures and materials. For example, in the context of bridges or buildings, engineers must ensure that materials can withstand shear stress to prevent failure or collapse.