Coulomb Friction Vs Viscous Friction:Comparative Analysis


Friction is a force of resistance that retards the relative motion of the matter. Coulomb friction and viscous friction are the sub-division of friction force. 

Though both Coulomb and viscous friction restrict the motion of the matter, they exhibit different natures. Comparing Coulomb friction vs. viscous friction leads to understanding how friction changes its properties with the matter.

Coulomb friction Vs Viscous friction

Both difference and similarities between coulomb and the viscous friction can be understood by comparative analysis. The below table gives the study of Coulomb friction vs. viscous friction.

Coulomb FrictionViscous Friction
Coulomb friction is applicable for stiction (static friction).Viscous friction is the another name for fluid friction.
Mass and volume are not the functions involved in the Coulomb friction.In the case of Viscous friction, only the volume is considered; it does not depend on the mass.
Coulomb friction is zero when the speed of the surface is zero, and it moves proportional to non-zero for all values of speed.Viscous friction directly corresponds to the speed at which the fluid flows.
The distance between the two contact surfaces makes a huge contribution to the Coulomb friction.Since viscous friction depends on the velocity gradient, it is also influenced by the distance between the layers of the fluid.
The change in the temperature has a direct effect on the Coulomb friction.The viscous friction is inversely influenced by the change in the temperature of the fluid.
It completely depends on the coefficient of friction of the surface.The factor coefficient of friction does not involve in this process.
Due to the Coulomb friction evolved between the surfaces may produce heat in some cases.Since viscous friction evolved between the two layers, it is completely an internal property and does not produce heat.
The occurrence of Coulomb friction is due to the adhesive force on the solid surface.Cohesive force is responsible for the viscous force to occur.
The variation of Coulomb friction is due to only the relative motion between the two surfaces.The deformation or movement of fluid layers in the viscous friction is due to the shear stress.

Definition of Coulomb Friction

Coulomb friction is a synonym for “dry friction.” It is exerted on the solid surface to restrict the lateral motion.

coulomb friction vs viscous friction
Image credits: Image by Rudy and Peter Skitterians from Pixabay 

Dry friction was named coulomb friction after Charles Augustin de Coulomb proposed a model to calculate dry friction. He gives the expression of inequality as

Ff µFn

Where; Ff is the friction force exerting between the two surfaces.

µ is the coefficient friction force. Fn is the normal force acting between the two surfaces.

Definition of viscous friction

Viscous friction is applied on the layers of both liquid and gas to restrict the relative flow of the fluid.

Image credits: Image by PublicDomainPictures from Pixabay 

Frequently Asked Questions.

What are the factors that affect the coefficient of friction?

The factors that may affect the coefficient of friction are

  • The nature of the surface – for the smooth surfaces, the friction is less. But in the case of rough surfaces, the coefficient of friction is more compared to the smooth surface.
  • Temperature – the change in temperature and the coefficient friction are corresponding to each other. At higher temperatures, the coefficient of friction is also high.

What do you meant by cohesive and adhesive forces?

These are two different forces exerted on the substance can be define as below.

  • Cohesive force is a tendency of a substance that makes two particles of the same substance hold together by creating a force of attraction.
  • Adhesive force is a tendency of two dissimilar surfaces attracting one another to keep in contact with each other.

Does the coefficient of friction is applied to the viscous friction?

As the coefficient of friction is a dimensionless quantity and the viscous friction has a certain dimension. So coefficient friction is not applicable to viscous friction.

A quantity coefficient of viscosity is used to measure viscous friction. It is the ratio of shearing stress acting between the layers of the fluid and the relative velocity at which the fluid is flowing.

 Does the pressure affect the viscous friction?

Under normal conditions, pressure does not affect the viscous friction acting on the fluid.

At high pressure, the intermolecular distance between the layers of the fluid decreases so that force between the molecules increases. This decreases the relative velocity between the two layers. This increases viscous friction.

A wooden block is sliding on the surface. Calculate the friction acting between the block and the surface if the normal force acting between them is 7N, and the coefficient of friction is 0.30.

Solution:

Given: Coefficient of friction µ = 0.30

             Normal force FN = 7N

The friction acting between the wooden block and the surface is given by

F = µFN

F = 0.30 × 7

F = 2.1 N

The Coulomb friction acting on an object is 5.6N, and the normal force acting on the object is 8.1N. Find the coefficient of friction.

Solution:

               Given: The friction force acting on the object is 5.6N

               The Normal force acting between object and surface is 8.1N

The Coulomb friction acting on the object is given by

Ff = µFn

5.6 = µ (8.1)

µ = 0.69

Can friction coefficient have a negative value?

Practically it is impossible for an object to have a negative value of the coefficient of friction.

In 2012, a study on the potential for friction demonstrated that for a low-load system, the normal force acting on the object and surface decreases which leads to an increase in the friction between the object and the surface. However, it contradicts the real-world experiment.

How does the distance between the two layers affect the viscous friction?

The distance between the two layers of the fluid directly corresponds to the change in the velocity between the two layers. It influences the viscous friction of the fluids.

When the fluid flows, the layers of the fluid exhibit distinct velocities. If the layers and the boundary of the fluid are closer, the shear stress acting at the boundary becomes maximum; this makes the velocity decrease. So the distance between the layers influences the viscous friction.

Keerthi Murthi

I am Keerthi K Murthy, I have completed post graduation in Physics, with the specialization in the field of solid state physics. I have always consider physics as a fundamental subject which is connected to our daily life. Being a science student I enjoy exploring new things in physics. As a writer my goal is to reach the readers with the simplified manner through my articles. Reach me – keerthikmurthy24@gmail.com

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