# Do we have viscous force acting between two layers

Frictional force between solids operates even when they do not move with respect to each other. Do we have viscous force acting between two layers even if there is no relative motion?

• wikipedia will be of help Oct 21, 2012 at 6:25

Do we have viscous force acting between two layers even if there is no relative motion?

No. From the Wikipedia article on viscosity:

In general, in any flow, layers move at different velocities and the fluid's viscosity arises from the shear stress between the layers that ultimately opposes any applied force

When the fluid is stationary, there's no velocity gradient, hence no shear stress.

• It is fantastic that so many answers to fluid mechanics can be addressed by just paying close attention to the various terms in the Navier Stokes' equations. To this day this equation and its breadth and depth amazes me. Oct 21, 2012 at 22:27
• One does not need to know about Navier Stokes equation to understand viscosity in fluids. Although it does help explain the effect and interaction of viscous forces, those things can be easily understood without knowing about the equation. Nov 10, 2012 at 3:15

Frictional force between solids operates even when they do not move with respect to each other.

There's a little confusion here. Force doesn't operate. It's either applied or not.

Suppose my car, which weighs about 2000 lb, has its brakes locked, so it can't roll. Suppose the friction coefficient of the rubber tires on the pavement is about 0.5. What does that mean?

That means if I want to shove it forward I need to push it forward with more than 1000 lb force, to overcome friction. If I only push with 500 lb force, then only 500 lb force is applied (and it won't move). If I push with 0 lb force (i.e. not push), then 0 lb force is applied (and of course it won't move).

So when solids do not move with respect to each other, it does not mean that frictional force is operating.