# What is the force that acts tangential to fluid's surface

1. Jan 24, 2010

### jarman007

In my book it is written that:
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.

Can anyone tell me what is the force that acts tangential to fluid's surface

I dont see any force tangentially acting on the fluid

2. Jan 24, 2010

### blkqi

Re: Fluids

Any force which acts on the face of a fluid can be decomposed into perpendicular and tangential components. Just line up your horizontal axis with the face of the fluid. If there exists a tangential component (on the horizontal axis), the shear stress will cause the fluid to distort, until just the perpendicular component remains; only then the fluid can be at rest.

Suppose you spill milk on your floor. The milk immediately spreads out to reduce the force of gravity acting on it tangentially. The milk spans out perpendicular to gravity. It will finally come to rest when the tangential force is minimal, balanced by the surface tension of the fluid.

The reason you cannot immediately grasp a force acting tangential to a fluid is exactly for this reason. Any fluid at rest has already spread out until something else (say, the walls of a beaker) supports its faces tangential to the gravity/normal forces.

In short, all this means is that a fluid cannot support itself in any shape like a solid can.

Hope this makes sense!

Last edited: Jan 24, 2010
3. Jan 25, 2010

### jarman007

Re: Fluids

Take initially the shape of milk be cuboid
Tell how gravity makes it spread

Gravity acts perpendicularly to horizontal surface so no sheer stress