In our consideration of Young's modulus and the Bulk modulus for solids, we examined how to describe how solids responded to normal forces squeezing them. But because solids have some rigidity, they can be deformed in another way by forces that are parallel to a surface, rather than perpendicular to it. We could do this using friction by squeezing another object to one side of our solid and trying to slide it. This is usually how it happens in the everyday world.
