It is well known that matter is divided into solids and fluids.
Fluids can be further divided into Liquids and Gases. It is taught
in schools, rightly so, that solids have a definite shape and a
definite size, while the liquids have a definite size, but no
definite shape. They assume the shape of the container they are
poured into. Gases on the other hand have neither a shape nor a
size. They can fill any container fully and assume its shape. But
we are engineers. We need a more precise definition. This comes
when we consider the response of a solid or a fluid to a shear
force. A solid resists a shear force while a fluid deforms
continuously under the action of a shear force.
Figure 1.1 Experiment to define a Fluid
A thought experiment is carried out to explain this further.
Consider two flat plates of infinite length placed a distance
h apart as shown in
Fig.1.2.
The lower plate is fixed while the upper plate is allowed to move.
Let us fill the gap in between the plates first with a solid substance. If now a shear force is applied to the upper plate the solid block deforms as shown. Line ab assumes a new position ab'and the upper plate is displaced by a distance bb'. The deformation produced is proportional to the applied shear stress F/A, where A is the area of the solid surface in contact with the plate.
Now let us fill the gap with a fluid, say water. What happens when a shear force is applied to the top plate? We find that it moves continuously ie., point b keeps moving and occupies positions b1, b2, b3, b4 etc at different instants of time. The fluid block between the plates deforms and continues to deform as long as the force is applied. This experiment shows that a fluid at rest cannot resist shear stress. Such an experiment also helps us to define viscosity, which we will take up later.
Figure 1.2,Deformation of
a Solid(a) and a Fluid (b) under the action of a shear force
