# Friction caused by a Fluid

1. Apr 11, 2005

I'm working on a fluid mechanics problem and Im not too sure how to approach it. There is a system with an inclined plane and 2 cubes, one attached to the other via a massless string and run through a massless pulley. One cube is dangeling down the vertical edge and the other is on the incline. They both weigh the same amount. The incine is 30 degrees and the question asks to calculate the tension in the string and the terminal velocity of the cubes. The incline is covered with a thin 0.1mm thick layer of oil which has a density of 886kg/m^3 and a dynamic viscosity of 0.29N s/m^2. The information it gives me is that the cubes specific gravity is 2.7 and the side dimentions are 20cm. The oil distribution can be considered to be linear.

I have it set up as a classical mechanics question, but not having any previous experience in fluids, I need to know what kind of force the fluid wil make with the Cube on the incline. I figure that there will be a shear stress. I have No idea how to calculate this. The rest of the problem I know how to set up. Is this shear stress calculated as a constant and then treated like kinetic friction, or is it considered a force? If someone could just quicky give me a quick explination and an equation or two on how to claculate the force exerted by the fluid, that would be great.

Regards,

2. Apr 11, 2005

### Speed

Viscosity = 256.94 Pas

Friction due to shear stress, F = 102776v

Cube mass, m = 21.6kg

$$m g (1 - Sin \theta) = 102776v$$
v = 4.0988ms-1

3. Apr 12, 2005

how did you get that viscosity? and the constant for the hear stress equation. If you could please expliain, I would really appreciate it.

Regards,

4. Apr 12, 2005

### Speed

You were given the kinematic viscosity, you need the simple viscosity, which is the kinematic viscosity multiplied by the density.

The shear shress equation is based on the definition of shear stress for a Newtonian fluid.
Tau = mu*v/z

5. Apr 12, 2005

so shear stress is in opposition to the motion, just like friction right. And could you also ezplain what the vriables are in the shar stress equation. I know the questions I'm asking are simplistic, but I need a definite answer. Thanks in advance.

Regards,

6. Apr 12, 2005

### Speed

Shear stress opposes relative motion in the fluid.

Tau = shear stress; mu = viscosity; v = velocity; z = thickness of layer.

Hope this helps.

7. Apr 13, 2005