Which term refers to force directed parallel to a surface?

• A. Rubbing
• B. Shear
• C. Pulling force
• D. Seal

Answer: (B)

A force directed along the plane of a surface is called a shear force. It tends to cause one part of a material to slide past another, acting parallel to the surface. This is what you’re identifying when you look at forces that try to shear through a joint or layer in a structure, which is a key consideration in bridge design and analysis. In contrast, a force that acts perpendicular to the surface is a normal force, which pushing or pulling straight into or away from the surface does not cause sliding along the plane. The other terms don’t fit because rubbing isn’t a standard technical term for a force direction, a pulling force is a general description that doesn’t specify orientation relative to the surface, and a seal has nothing to do with force direction.