in fluid mechanics when we study shear
we break up the fluid into little cubes
and on each of those cubes we break up
the shear into vectors of three
components each and each of those
components applies to one of the six
faces of a cube so we end up with
drawings like this one here where we
have every time on each of the six faces
of the cube three components of shear
and one of those is very strange it is a
component of shear every time that is
perpendicular to the area tau XX tau YY
and tau ZZ so how can this be how is it
possible that there is shear
perpendicular to a surface if I take
here a cube and this by approximation would
be a cube a plastic box if I try to
shear perpendicular to some area it's
not going to work I can shear in this
direction or that direction but how
could I shear through the side of the
cube the answer is on the sides of a
plastic box it's not possible to shear
through the surface so it doesn't work
that way but in the fluid when we take
this cube this cube is traversed by
fluid fluid flows through the cube it's
just an imaginary frame an imaginary
piece of volume that we identify inside
the fluid but the fluid passes through
and if the fluid passes through and it
has shear associated to it at that
point it is quite possible that the
shear points in a direction that has
some component that is perpendicular to
the plane of the cube
yes so if we look at this cube here when
we think about shear as it applies to
the mass of fluid that is inside the
cube we have to think that there is a
flow through each of the faces of that
cube it's not a solid box around which
the fluid will have to flow it is a
frame through the fluid will flow if the
fluid flows through then the shear
that's applying locally may well have a
component that's perpendicular to the
plane so here you are this is the story
behind tau xx
and the "shear perpendicular".
