A pressure sore is an ulceration of the skin
and/or deeper tissues due to unrelieved pressure,
shear force(s), and/or frictional force(s) . Shear
forces are produced when contiguous parts slide
relative to each other in a direction parallel to their
plane of contact . Frictional forces are due to the
resistance of one body sliding, rolling, or flowing
over another . Either of these forces can be produced
by a person sliding on bed sheets when being placed