When a body is held or rests in equilibrium in contact with a surface (e.g. a slope, the edge of a block etc.) how do we work out which direction the friction acts in? I'm used to the friction acting parallel to the surface i.e. slope but in one question I have, of a beam resting on the edge of a block, the friction seems to act along the beam (rather than along the face of the block). Why is this?