Understanding Shearing Stress & Tangential Forces on Fluids

[Rishabh Narula]

Homework Statement

"A fluid, in contrast to a solid, is a substance that can flow. Fluids conform to the
boundaries of any container in which we put them. They do so because a fluid
cannot sustain a force that is tangential to its surface. (In the more formal
language of Module 12-3, a fluid is a substance that flows because it cannot
withstand a shearing stress. It can, however, exert a force in the direction perpendicular to its surface.) Some materials, such as pitch, take a long time to conform
to the boundaries of a container, but they do so eventually; thus, we classify even
those materials as fluids"

Relevant Equations

none

what does it mean that fluid cannot sustain a force that is tangential to its surface.which surface fluids's surface or container?
and what does it mean it cannot withstand shearing stress.what's shearing stress and what does it mean here.and why exert force only in direction perpendicular to surface?can someone provide pictures or animations,hard to visualize what this para from halliday book means.

 

[BvU]

which surface fluids's surface or container?

fluid surface. The story simply explains that a fluid flows around any object that pushes that fluid

 

[scottdave]

So the boundary where the fluid touches the container is also the fluid surface. Also the horizontal "surface" if the container is not full is also the surface. Tangential is parallel to the surface. Put a canoe in the water and it glides parallel to the surface with little effort. Sure it sinnks into the water some and will move water aside as it moves forward, but the surface touching the canoe is sheared and moves along with the canoe.

 

[haruspex]

what does it mean that fluid cannot sustain a force that is tangential to its surface

The use of the word "sustain" is significant. It means that if you apply a force, no matter how small, tangential to the surface of the fluid then that part of the surface will move and continue to move for as long as the force is applied.
Contrast this with a solid, in which the surface will move a little, but as the movement progresses its resistance to movement increases, so the movement comes to a stop.