11 Contact Materials with Highest Coefficient of Friction

11 Contact Materials with Highest Coefficient of Friction

11 Contact Materials with the Highest Coefficient of Friction

written by Stanley Udegbunam || Mar 6th, 2021

materials with the highest coefficient of friction

We recently analyzed the coefficient of friction of 91 contact materials currently listed on our friction coefficient table.

And the findings?

We noticed that 30 contact materials have a known coefficient value ≥ 0.5 and 61 contact materials have a coefficient value less than 0.5

From the analysis, here is a list of the top 11 contact materials with the highest known coefficient of friction.

NoCONTACT MATERIALSCOEFFICIENT OF FRICTION
1.Solid on Rubber1.0 - 4.0
2.Silver on Silver1.4
3.Aluminium on Aluminium1.05 - 1.35
4.Platinum on Platinum1.2
5.Cast Iron on Cast Iron1.1
6.Copper on Cast Iron1.05
7.Copper on Copper1.0
8.Iron on Iron1.0
9.Steel on Lead0.95
10.Glass on Glass0.9 - 1.0
11Nickel on Nickel0.7 - 1.1
NoCONTACT MATERIALSCOEFFICIENT OF FRICTION
1.Solid on Rubber1.0 - 4.0
2.Silver on Silver1.4
3.Aluminium on Aluminium1.05 - 1.35
4.Platinum on Platinum1.2
5.Cast Iron on Cast Iron1.1
6.Copper on Cast Iron1.05
7.Copper on Copper1.0
8.Iron on Iron1.0
9.Steel on Lead0.95
10.Glass on Glass0.9 - 1.0
11Nickel on Nickel0.7 - 1.1

NOTE: It’s wrong to say ask questions like “what materials have the highest coefficient of friction?”

REASONS: Coefficient of friction (C.O.F) is a surface property and not an intrinsic property of individual materials.

This means that coefficient of friction can only exist between two materials in contact.

Let’s simplify this further.

Given 3 materials;

Material A, Material B, and Material C

You can’t talk about the coefficient of friction of Material A as “stand-alone” material.

But rather, the coefficient of friction of Material A on Material B or Material A on Material C.

More like a pair combination and it’s logical since friction itself is a contact force; meaning it must involve two contact surfaces.

Having this in mind, the right question will be:

what contact materials have the highest coefficient of friction? (or paraphrased in any form that signifies material pairs).

And the right answer is clearly outlined in the table above.

Contact Materials with the Highest C.O.F: HOW THE ANALYSIS WAS CARRIED OUT

Few days ago, we published a complete guide on coefficient of friction which contains a friction coefficient table that outlines the C.O.F of over 91 known contact materials.

We needed only materials with high coefficient of friction so we screened the list and selected only materials with coefficient of friction greater than 0.5.

About 30 contact materials fell into this limit.

A graph indicating the coefficient of friction on the y-axis and the contact materials on the x-axis was plotted.

Here’s the result:

Contact materials with coefficient of friction greater than 0.5

The graph clearly shows the peak points with the 11 highest coefficient of friction.

Some coefficient values exist in ranges (i.e. x-y).

For the purpose of the plot, the average value was calculated and used on the graph.

For example:

The coefficient of friction of solid on rubber is 1.0 – 4.0

The average coefficient is 2.5. This is a preferred value for graphical purposes only.

The chart below further outlines the 11 contact materials with the highest C.O.F in descending order.

11 Contact materials with the highest coefficient of friction

Further Observations:

Using the average values, we discovered that 8 of these contact materials have a friction coefficient ≥ 1.0

Contact Materials with friction coefficients equals or greater than 1.0

NoCONTACT MATERIALSCOEFFICIENT OF FRICTION
1.Solid on Rubber1.0 - 4.0
2.Silver on Silver1.4
3.Aluminium on Aluminium1.05 - 1.35
4.Platinum on Platinum1.2
5.Cast Iron on Cast Iron1.1
6.Copper on Cast Iron1.05
7.Copper on Copper1.0
8.Iron on Iron1.0
NoCONTACT MATERIALSCOEFFICIENT OF FRICTION
1.Solid on Rubber1.0 - 4.0
2.Silver on Silver1.4
3.Aluminium on Aluminium1.05 - 1.35
4.Platinum on Platinum1.2
5.Cast Iron on Cast Iron1.1
6.Copper on Cast Iron1.05
7.Copper on Copper1.0
8.Iron on Iron1.0

This accounts for roughly 73% of our list with only 27% falling below coefficient value of 1.0

Contact Materials with coefficient of friction greater than 1

Contact materials with such high friction coefficient value are required in automobile industries and engineering components to:

  1. Quickly bring a body to rest
  2. Avoid motion of specific parts. 

QUESTION: What contact materials have the highest coefficient of friction?

ANSWER: Solid on Rubber (1.0 – 4.0)

What are the factors affecting the coefficient of friction?

What are the factors affecting the coefficient of friction?

What are the factors affecting the coefficient of friction?

written by Stanley Udegbunam || March 6th, 2021

Factors affecting the coefficient of friction

There are only two factors that affect the coefficient of friction and they are:

  1. The nature of the surfaces in contact.
  2. The friction type (Static, kinetic, or rolling friction)

The coefficient of friction is a surface property. Therefore, its value is constant for a given combination of material surfaces.

The coefficient of friction for given pair of surfaces only varies across different surface conditions like wet, dry, and greasy surfaces.

Nevertheless, these variations are known for different surface pairs through experimental analysis and specified on a friction coefficient table.

direction of static friction guide

How does the nature of contact surfaces affect the coefficient of friction?

Primarily, friction is caused by the interlocking of tiny surface irregularities known as asperity.

The extent of asperity interlocking varies according to the nature of surfaces in contact.

This means that different surface pairs will either possess a higher or lower friction resistance based on their individual surface property.

As a result of this, the coefficient of friction will be dependent on the nature of contact surface under consideration.

surface asperity affects the coefficient of friction

It’s worth knowing that all objects have microscopic rough planes regardless of how smooth or polished they may appear.

Nature of surface affects coefficient of static friction

NOTE: Coefficient of friction does not exist for a single material but rather for a given pair of surfaces in contact.

Therefore, it’s wrong to say things like: “What’s the coefficient of friction of a rubber chair”

Friction is a contact force, it must involve 2 interacting surfaces, same principle applies to friction coefficients.

Having this in mind, a better question phrase to the format described above will be:

What’s the coefficient of friction of a rubber chair on a concrete floor?

What’s the coefficient of friction of a rubber chair on a rubber tile?

Sounds better now right? (you can just nod in agreement).

The answer is provided in the table below.

MATERIAL COMBINATIONStatic Coefficient of Friction (Dry Surface)
Rubber on Concrete1.0
Rubber on tile1.16
MATERIAL COMBINATIONStatic Coefficient of Friction (Dry Surface)
Rubber on Concrete1.0
Rubber on tile1.16

This table is extracted from Afrilcate’s guide on Static friction coefficient, you can see the complete table here:

Comprehensive List of Static friction Coefficient

Variations of Contact Surfaces with Coefficient of Friction

As mentioned earlier, the coefficient of friction varies across different pairs of surfaces.

Also for a given surface pair, the coefficient changes according to the surface condition.

Surface condition means either wet, dry or greasy surfaces.

Coefficient of friction of Glass on Glass

For Dry surfaces:

the coefficient of friction of glass on glass in a stationary position is 0.94

For Wet surfaces:

the coefficient of friction of glass on glass in a stationary position ranges from 0.1 – 0.6

Coefficient of friction of glass on glass

 Coefficient of friction of Leather on Wood

The Coefficient of friction of leather in a stationary position ranges from 0.3 – 0.4

coefficient of friction leather on wood.

Coefficient of friction of Steel on Steel

For wet surfaces:

The coefficient of friction of steel on steel  ranges from 0.09 – 0.19

For dry surfaces:

the coefficient of friction is 0.74

coefficient of friction of steel on steel

How does the type of friction affect the coefficient of friction?

Generally, there are three types of friction:

  1. Static friction: The friction force between two surfaces at rest relative to each other.
  2. Kinetic friction: The friction between two surfaces that moves or slides relative to each other.
  3. Rolling friction: The friction force that resists the rolling motion of an object against a surface. Also known as rolling resistance or rolling drag.

These three friction types possess friction coefficients known as the coefficient of static friction, the coefficient of kinetic friction, and the coefficient of rolling friction respectively where:

Coefficient of static friction: is the numerical value that quantifies the degree of stickiness between a static object and its contact surface.

Coefficient of kinetic friction: is the numerical value that quantifies the friction intensity of a sliding surface relative to another.

Coefficient of rolling friction: is a numerical value that quantifies the degree of motion resistance between a surface and a rolling object.

The static friction coefficient has a higher value when compared to other friction types for a given set of contacting surfaces.

This is because the prolonged/undisturbed state of static position allows more time for asperity locking and molecular bond formation unlike objects in the sliding or rolling state.

That’s why it’s much easier to keep a moving object in motion than to initiate the motion of the object from rest.

Again, the coefficient of rolling friction has the least value when compared to other friction coefficient types for a given set of contacting surfaces.

Coefficient of friction of different surfaces

Variation of friction types with Coefficient of friction

MATERIAL (dry surfaces)Coefficient of Static FrictionCoefficient of Kinetic FrictionCoefficient of Rolling Friction
Steel on Steel0.740.570.001 - 0.002
Rubber on Concrete1.0 – 4.00.6 - 0.850.35 - 0.45
Rubber on Asphalt1.0 – 3.50.5 - 0.80.02
Aluminium on Steel0.610.47-
Copper on Steel0.530.36-
Wood on Wood0.25 – 0.50.2-
Glass on Glass0.940.4-
Teflon on Teflon0.040.04-
Cast Iron on Cast Iron1.10.15-
Copper on cast Iron1.050.29-
Zinc on cast iron0.850.21-
MATERIAL (dry surfaces)Coefficient of Static FrictionCoefficient of Kinetic FrictionCoefficient of Rolling Friction
Steel on Steel0.740.570.001 - 0.002
Rubber on Concrete1.0 – 4.00.6 - 0.850.35 - 0.45
Rubber on Asphalt1.0 – 3.50.5 - 0.80.02
Aluminium on Steel0.610.47-
Copper on Steel0.530.36-
Wood on Wood0.25 – 0.50.2-
Glass on Glass0.940.4-
Teflon on Teflon0.040.04-
Cast Iron on Cast Iron1.10.15-
Copper on cast Iron1.050.29-
Zinc on cast iron0.850.21-

Coefficient of Rolling Friction (Fully Explained)

Coefficient of Rolling Friction (Fully Explained)

Coefficient of Rolling Friction (Fully Explained)

written by Stanley Udegbunam ||updated Oct 23, 2020

Overview

What is Coefficient of rolling friction?

The coefficient of rolling friction is a numerical value that quantifies the degree of motion resistance between a surface and a rolling object.

Also known as rolling friction coefficient, it measures the rolling resistance of an object against a surface.

A high coefficient of rolling friction indicates high rolling resistance between the given contact surfaces.

And a low coefficient of rolling friction indicates low rolling resistance between the given contact surfaces.

Recall that there are three types of dry friction: Static friction, kinetic friction, and rolling friction.

While static friction possesses the highest friction force, rolling friction accounts for the least friction force.

This is due to the minimal contact area of the rolling object and the surface beneath.

rolling friction coefficient

 

As a result of this, the coefficient of rolling friction usually have the least value when compared with the coefficient of other friction types.

 

Static Friction Coefficient

Want to know more about rolling friction?, see our in-depth guideRolling Friction (The Complete Guide)

Formula for Coefficient of rolling friction

Rolling friction coefficient is denoted with µk,r.

The formula for the coefficient of kinetic friction is given by;

µk,r =  Fk,r /N

Where,

  • Fk,r is the force of rolling friction
  • N is the normal force, sometimes it’s denoted by η

The Normal force has the same magnitude as the weight.

It’s represented by the equation; N = m x g

If the object or plane is inclined at an angle (∅), the normal force becomes:

N = m gcos∅

Where m = mass of the object.

And g = acceleration due to gravity.

F and N are measured in units of force (such as Newton or pound).

Since friction is a force, the unit of the frictional force is the newton (N).

The coefficient of rolling friction is unitless.

How to find the Coefficient of rolling friction

To find the coefficient of rolling friction, we will use the formula above to carry out some friction coefficient calculations.

Calculations on Coefficient of rolling friction

Example 1

A loaded trolley pushed on a wooden floor with μk,r = 0.2 has a mass of 40 kg.

What is the rolling resistance? Take g = 9.81m/s2

Solution

Fk,r​​k,r​ Fn

Normal force, Fn​ = mg

Coefficient of friction, μ k,r= 0.2

Fk,r​​k,r x mg

Fk,r​​= 0.2 x 40 x 9.81 = 78.48 N

Example 2

Consider a milk tin rolling the floor.

Given N = 83 N and

Fk,r = 2.76 N,

What is the coefficient of the rolling friction?

Solution

Fk,r = µk,rN

µk,r = Fk,r/N

µk,r  = 2.76/83 = 0.033

TEST YOUR KNOWLEDGE

Consider a car tire rolling on a smooth horizontal surface.

Given N = 806 N and

Fk,r = 1.38 N, the coefficient of the rolling friction is:

a.) 0.0130

X wrong!!

b.) 0.0017

✔ Correct

c.) 0.020

X wrong!!

d.) none of the above

X wrong!!

Comprehensive List of Rolling friction Coefficients

The list below contains different rolling friction coefficient of various materials and surfaces in contact.

Because of the various factors affecting rolling friction, the coefficient of rolling friction is usually determined experimentally.

COEFFICIENT OF ROLLING FRICTION OF DIFFERENT MATERIAL/CONTACT SURFACE
railroad steel wheels on steel rails
0.001 - 0.002

rubber on concrete

0.35 - 0.45

nylon on concrete

0.03 - 0.04

bicycle tire on wooden track

0.001

bicycle tire on concrete

0.002

bicycle tire on asphalt road

0.004

bicycle tire on rough paved road

0.008

truck tire on asphalt

0.006 - 0.01

car tires on concrete, new asphalt, cobbles small new

0.01 - 0.015

car tires on tar or asphalt

0.02

car tires on gravel - rolled new

0.02

car tires on cobbles  - large worn

0.03

car tires on solid sand, gravel loose worn, soil medium hard

0.04 - 0.08

car tires on loose sand

0.2 - 0.4

Polyurethane

0.04 - 0.08

CONTACT SURFACES

COEFFICIENT OF ROLLING FRICTION

railroad steel wheels on steel rails
0.001 - 0.002

rubber on concrete

0.35 - 0.45

nylon on concrete

0.03 - 0.04

bicycle tire on wooden track

0.001

bicycle tire on concrete

0.002

bicycle tire on asphalt road

0.004

bicycle tire on rough paved road

0.008

truck tire on asphalt

0.006 - 0.01

car tires on concrete, new asphalt, cobbles small new

0.01 - 0.015

car tires on tar or asphalt

0.02

car tires on gravel - rolled new

0.02

car tires on cobbles  - large worn

0.03

car tires on solid sand, gravel loose worn, soil medium hard

0.04 - 0.08

car tires on loose sand

0.2 - 0.4

Polyurethane

0.04 - 0.08

table extracted from engineering toolbox rolling resistance

Frequently Asked Questions on Rolling Friction Coefficient

A short collected of common FAQ on the coefficient of rolling friction from the web.

The answers to the questions are all provided in the table above.

1. What’s the coefficient of rolling friction between a car tire and road surface?

Ans: The coefficient of rolling friction between car tires and road surface ranges from 0.01 – 0.02 depending on the nature of the road surface and the threaded nature of the tire.

Smooth/Slick tire offers better traction on dry surfaces and smooth roads.

While threaded tire offers better traction on wet surfaces and coarse/rough roads.

coefficient of rolling friction of car tire

 

rolling resistance of car tires

2. What’s the coefficient of rolling friction of steel and steel?

Ans: The coefficient of rolling friction of steel and steel ranges from 0.001 – 0.002.

3. What is the coefficient of rolling friction of rubber on concrete?

Ans: The coefficient of rolling friction of rubber on concrete ranges from 0.35 to 0.45. 

4. What is the coefficient of rolling friction of nylon on concrete.

Ans: The coefficient of  rolling friction of nylon on concrete ranges from 0.03 to 0.04 

5. What is the coefficient of rolling friction of Polyurethane?

Ans: The coefficient of rolling friction of Polyurethane ranges from 0.04 to 0.08 (refer to table)

 

Now…Over to you

What do you think about this article?

was it helpful in any way and would you like to see more content like this?

Share your thoughts and answers with us in the comment section below.

Coefficient of Kinetic Friction (Fully Explained) – Afrilcate

Coefficient of Kinetic Friction (Fully Explained) – Afrilcate

Coefficient of Kinetic Friction (Fully Explained)

written by Stanley Udegbunam ||updated Oct 22, 2020

Overview

What is Coefficient of kinetic friction?

The coefficient of kinetic friction is a numerical value that quantifies the friction intensity of a sliding surface relative to another.

Also known as kinetic friction coefficient, it measures the extent of material grip between a moving object and its contact surface.

Recall that there are three types of dry friction: Static friction, kinetic friction, and rolling friction.

Although the coefficient of kinetic friction is higher than of rolling friction, it’s still less than the coefficient of static friction.

This is because the force of static friction is greater than both kinetic and rolling friction.

Therefore more force is required to initiate motion when an object is at rest but once the motion is initiated, less force will be required to maintain the motion.

Rolling objects usually have the least friction coefficient due to its minimal contact area.

 

Static Friction Coefficient

Formula for Coefficient of kinetic friction

Kinetic friction coefficient is denoted with µs.

The formula for the coefficient of kinetic friction is given by;

µk Fk/N

Where,

  • Fis the force of kinetic friction
  • N is the normal force, sometimes it’s denoted by η

The Normal force has the same magnitude as the weight.

It’s represented by the equation; N = m x g

If the object or plane is inclined at an angle (∅), the normal force becomes:

N = m gcos∅

Where m = mass of the object.

And g = acceleration due to gravity.

F and N are measured in units of force (such as Newton or pound).

Since friction is a force, the unit of the frictional force is the newton (N).

The coefficient of kinetic friction is unitless.

How to find the Coefficient of kinetic friction

To find the coefficient of kinetic friction, we will use the formula above to carry out some friction coefficient calculations.

Calculations on Coefficient of kinetic friction

Example 1

A total force of 20N is required to move a 5 kg block along a horizontal wooden surface.

If the acceleration due to gravity is estimated to be 10 m/s2, calculate the coefficient of kinetic friction.

solution

Friction force (Fk ) = 20N

mass (m) = 5kg

g = 10 m/s2

Fk = µk x N

µk = F/ N

µk = Fk/m x g = 20/(5×10) = 20/50 = 0.4

therefore the coefficient of kinetic friction is 0.4

 

Example 2

A 3kg  stainless cup is pushed across a horizontal glass table with µk = 0.6

Calculate the kinetic friction force given that g = 9.81 m/s2.

Solution

Fk = µkN

but N = m x g

Fk = µk mg

 Fk = 0.6 x 3 x 9.81

Fk = 17.66 N

TEST YOUR KNOWLEDGE

The force acting on a moving train is 280N.

If the kinetic friction is 115N, determine the kinetic friction coefficient.

a.) 0.32

X wrong!!

b.) 0.41

✔ Correct

c.) 0.57

X wrong!!

d.) none of the above

X wrong!!

Comprehensive List of Kinetic friction Coefficients

The list below is a long list of kinetic friction coefficients of different materials measured under different surface conditions.

The greasy surface condition represents any form of lubrication.

C.O.K-F —- Coefficient of Kinetic friction

Scroll sideways to view the complete list

Material 1

Material 2

Coefficient of Kinetic Friction

DRY

GREASY

Aluminum

Aluminum

1.4

-

Aluminum

Mild Steel

0.47

-

Brass

Cast Iron

0.3

-

Bronze

Cast Iron

0.22
-

Cadmium

Mild Steel

0.46

-

Cast Iron

Cast Iron

0.15

0.07

Cast Iron

Oak

0.49

0.075

Copper

Cast Iron

0.29

-

Copper

Mild Steel

0.36

0.18

 Copper

Steel (304 stainless)

0.21

-

Copper

Steel

0.8

-

Glass

Glass

0.4

0.09 - 0.12

Glass

Nickel

0.56

-

Lead

Cast Iron

0.43

-

Lead

Steel

1.4

-

Nickel

Nickel

0.53

0.12

Nickel

Mild Steel

0.64

0.178

Oak

Oak (parallel grain)

0.48

-

Oak

Oak (cross grain)

0.32

0.072

Rubber

Asphalt (Dry)

0.5 - 0.8

-

Rubber

Asphalt (Wet)

0.25- 0.75

-

Rubber

Concrete (Dry)

0.6 - 0.85

-

Rubber

Concrete (Wet)

0.45 - 0.75

-

Steel(Mild)

Brass

0.44

-

Steel (Mild)

Cast Iron

0.23
0.133

Steel

Copper Lead Alloy

-

0.145

Steel (Mild)

Lead

0.95

0.3

Steel (Mild)

Phos. Bros

0.34

0.173

Steel (Mild)

Steel (Mild)

0.57

0.09-0.19

Steel(Hard)

Steel (Hard)

0.42

0.029-0.12

Teflon

Steel

-

0.04

Teflon

Teflon

-

0.04

Tin

Cast Iron

0.32

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Aluminium Alloy 6061-T6

0.38

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Titanium Alloy Ti-6Al-4V(Grade 5)

0.30

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Bronze

0.27

-

Zinc 

Cast Iron

0.21

-

MATERIAL 1

MATERIAL 2

C. O. K-F (DRY)

C. O. K-F(GREASY)

Aluminum

Aluminum

1.4

-

Aluminum

Mild Steel

0.47

-

Brass

Cast Iron

0.3

-

Bronze

Cast Iron

0.22
-

Cadmium

Mild Steel

0.46

-

Cast Iron

Cast Iron

0.15

0.07

Cast Iron

Oak

0.49

0.075

Copper

Cast Iron

0.29

-

Copper

Mild Steel

0.36

0.18

 Copper

Steel (304 stainless)

0.21

-

Copper

Steel

0.8

-

Glass

Glass

0.4

0.09 - 0.12

Glass

Nickel

0.56

-

Lead

Cast Iron

0.43

-

Lead

Steel

1.4

-

Nickel

Nickel

0.53

0.12

Nickel

Mild Steel

0.64

0.178

Oak

Oak (parallel grain)

0.48

-

Oak

Oak (cross grain)

0.32

0.072

Rubber

Asphalt (Dry)

0.5 - 0.8

-

Rubber

Asphalt (Wet)

0.25- 0.75

-

Rubber

Concrete (Dry)

0.6 - 0.85

-

Rubber

Concrete (Wet)

0.45 - 0.75

-

Steel(Mild)

Brass

0.44

-

Steel (Mild)

Cast Iron

0.23
0.133

Steel

Copper Lead Alloy

-

0.145

Steel (Mild)

Lead

0.95

0.3

Steel (Mild)

Phos. Bros

0.34

0.173

Steel (Mild)

Steel (Mild)

0.57

0.09-0.19

Steel(Hard)

Steel (Hard)

0.42

0.029-0.12

Teflon

Steel

-

0.04

Teflon

Teflon

-

0.04

Tin

Cast Iron

0.32

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Aluminium Alloy 6061-T6

0.38

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Titanium Alloy Ti-6Al-4V(Grade 5)

0.30

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Bronze

0.27

-

Zinc 

Cast Iron

0.21

-

table extracted from internet archive

Frequently Asked Questions on Kinetic Friction Coefficient

1. What’s the coefficient of kinetic friction of wood on wood?

Ans: 0.2 (refer to the table above)

2. What’s the coefficient of kinetic friction between rubber and asphalt?

Ans: The coefficient of kinetic friction between rubber and asphalt ranges between 0.5 – 0.8 (dry asphalt) and 0.25-0.75 (wet asphalt)

Ans: 0.25 – 0.5 (refer to table above)

3. What is the coefficient of kinetic friction between steel and teflon?

Ans: The coefficient of steel and teflon is 0.04 for greasy surface condition. (refer to table)

4. What is the coefficient of kinetic friction between glass and nickel.

Ans: The coefficient of  kinetic friction between glass and nickel is 0.56

 

Now…Over to you

What do you think about this article?

was it helpful in any way and would you like to see more content like this?

Share your thoughts and answers with us in the comment section below.

Coefficient of Static Friction (Fully Explained)

Coefficient of Static Friction (Fully Explained)

Coefficient of Static Friction (Fully Explained)

written by Stanley Udegbunam ||updated Oct 20, 2020

Overview

What is Coefficient of static friction?

The coefficient of static friction is a numerical value that quantifies the degree of stickiness between a static object and its contact surface.

It is the ratio of the Static friction between two contact surfaces to the normal force acting on the body.

Recall that there are three types of dry friction: Static friction, kinetic friction and rolling friction.

The coefficient of static friction is typically higher than the coefficient of kinetic friction and the rolling friction coefficient.

This is because static friction is higher than kinetic friction and rolling friction.

This implies that more force will be required to initiate motion if an object is stationary.

Since kinetic friction is higher than rolling friction, the Coefficient of rolling friction usually have the least friction coefficient.

 

Static Friction Coefficient

Formula for Coefficient of static friction

Static friction coefficient is denoted with µs.

The formula for the coefficient of static friction is given by;

µs Fs/N

Where,

  • Fis the force of static friction
  • N is the normal force, sometimes it’s denoted by η

The Normal force has the same magnitude as the weight.

It’s represented by the equation; N = m x g

If the object or plane is inclined at an angle (∅), the normal force becomes:

N = m gcos∅

Where m = mass of the object.

And g = acceleration due to gravity.

F and N are measured in units of force (such as Newton or pound).

Since friction is a force, the unit of the frictional force is the newton (N).

The coefficient of static friction is unitless.

How to find the Coefficient of static friction

To find the coefficient of static friction, we will use the formula above to carry out some friction coefficient calculations.

Calculations on Coefficient of static friction

Example 1

The force acting on a table-top microwave is 220N.

If the value of the static frictional force is 506N, determine the static friction coefficient.

Fs = µs N

µs = Fs/N

µs = 506/220 = 2.3

Example 2

Calculations on Static Friction Coefficient

A 50N force acts on a kerosene lamp as shown in the diagram. If the static friction is 15N, calculate the coefficient of static friction.

Solution

µs = Fs/N

Static friction = 15N

Normal force, N = 50 N

Coefficient of friction, µs = 15/50 = 0.3

µs = 0.3

 

Calculations on Static Friction Coefficient

TEST YOUR KNOWLEDGE

The force acting on a standing electric fan is 220N.

If the value of the static frictional force 125N, determine the static friction coefficient.

a.) 0.32

X wrong!!

b.) 1.11 x 105

X wrong!!

c.) 0.57

✔ Correct

d.) none of the above

X wrong!!

Comprehensive List of Static friction Coefficients

The list below is a long list of static friction coefficients of different materials measured under different surface conditions.

The greasy surface condition represents any form of lubrication.

C.O.S-F —- Coefficient of Static Friction

Scroll sideways to view the complete list

Material 1

Material 2

Coefficient of Static Friction

DRY

GREASY

Aluminum

Aluminum

1.05-1.35

0.3

Aluminum

Mild Steel

0.61

-

 Brake Material

Cast Iron

0.4

-

Brake Material

Cast Iron (Wet)

0.2

-

Brick 

Wood

0.6

-

Bronze

Steel

-
0.16

Cadmium

Cadmium

0.5

0.05

Cast Iron

Cast Iron

1.1

-

Chromium

Chromium

0.41

0.34

Copper

Cast Iron

1.05

-

Copper

Copper

1.0

0.08

Copper

Mild Steel

0.53

-

 Copper

Steel (304 stainless)

0.23

-

Copper-Lead Alloy

Steel

0.22

-

Diamond

Diamond

0.1

0.05-0.1

Diamond

Metal

0.1 -0.15

0.1

Glass

Glass

0.9 - 1.0

0.1-0.6

Glass

Metal

0.5 - 0.7

0.2-0.3

Glass

Nickel

0.78

-

Graphite

Graphite

0.1

0.1

Graphite

Steel

0.1

0.1

Graphite (In vacuum)

Graphite (In vacuum)

0.5 - 0.8

-

Hard Carbon

Hard Carbon

0.16

0.12-0.14

Hard Carbon

Steel

0.14

0.11-0.14

Iron

Iron

1.0

0.15-0.2

Leather

Wood

0.3 - 0.4

-

Leather

Metal(Clean)

0.6

0.2

Leather

Metal(Wet)

0.4

-

Leather

Oak (Parallel grain)

0.61

-

Magnesium

Magnesium

0.6

0.08

Nickel

Nickel

0.7-1.1

0.28

Nylon

Nylon

0.15 - 0.25

-

Oak

Oak (parallel grain)

0.62

-

Oak

Oak (cross grain)

0.54

-

Platinum

Platinum

1.2

0.8

Plexiglas

Plexiglas

0.8

0..4-0.5

 Plexiglas

Steel

0.4 - 0.5

0.5

Polystyrene

Polystyrene

0.5

0.3-0.35

Polystyrene

Steel

0.3-0.35

0.2

Rubber

Rubber

1.16

-

Saphire

Saphire

0.2

0.2

Silver

Silver

1.4

0.55

Sintered Bronze

Steel

-

0.13

Solids

Rubber

1.0 - 4.0

-

Steel

Aluminium Bros

0.45

-

Steel

Brass

0.35

0.19

Steel(Mild)

Brass

0.51

-

Steel (Mild)

Cast Iron

-
0.183

Steel

Cast Iron

0.4

-

Steel

Copper Lead Alloy

0.22

0.145

Steel (Hard)

Graphite

0.21

-

Steel

Graphite

0.1

-

Steel (Mild)

Lead

0.95

0.3

Steel (Mild)

Phos. Bros

-

0.173

Steel

Phos Bros

0.35

-

Steel(Hard)

Polythened

0.2

-

Steel(Hard)

Polystyrene

0.3-0.35

-

Steel (Mild)

Steel (Mild)

0.74

0.09-0.19

Steel(Hard)

Steel (Hard)

0.78

0.029-0.12

Steel

Zinc (Plated on steel)

0.5

-

Teflon

Steel

0.04

0.04

Teflon

Teflon

0.04

0.04

Titanium Alloy Ti-6Al-4V(Grade 5)

Aluminium Alloy 6061-T6

0.41

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Titanium Alloy Ti-6Al-4V(Grade 5)

0.36

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Bronze

0.36

-

Tungsten Carbide

Tungsten Carbide

0.2-0.25

0.12

Tungsten Carbide

Steel

0.4 - 0.6

0.08-0.2

Tungsten Carbide

Copper

0.35

-

Tungsten Carbide

Iron

0.8

-

Wood

Wood(clean)

0.25 - 0.5

-

Wood

Wood (Wet)

0.2

-

Wood

Metals(Clean)

0.2-0.6

-

Wood

Metals (Wet)

0.2

-

Wood

Brick

0.6

-

Wood

Concrete

0.62

-

Zinc

Zinc

0.6

0.04

Zinc 

Cast Iron

0.85

-

MATERIAL 1

MATERIAL 2

C.O.S.F (Dry)

C.O.S.F (Greasy)

Aluminum

Aluminum

1.05-1.35

0.3

Aluminum

Mild Steel

0.61

-

 Brake Material

Cast Iron

0.4

-

Brake Material

Cast Iron (Wet)

0.2

-

Brick 

Wood

0.6

-

Bronze

Steel

-
0.16

Cadmium

Cadmium

0.5

0.05

Cast Iron

Cast Iron

1.1

-

Chromium

Chromium

0.41

0.34

Copper

Cast Iron

1.05

-

Copper

Copper

1.0

0.08

Copper

Mild Steel

0.53

-

 Copper

Steel (304 stainless)

0.23

-

Copper-Lead Alloy

Steel

0.22

-

Diamond

Diamond

0.1

0.05-0.1

Diamond

Metal

0.1 -0.15

0.1

Glass

Glass

0.9 - 1.0

0.1-0.6

Glass

Metal

0.5 - 0.7

0.2-0.3

Glass

Nickel

0.78

-

Graphite

Graphite

0.1

0.1

Graphite

Steel

0.1

0.1

Graphite (In vacuum)

Graphite (In vacuum)

0.5 - 0.8

-

Hard Carbon

Hard Carbon

0.16

0.12-0.14

Hard Carbon

Steel

0.14

0.11-0.14

Iron

Iron

1.0

0.15-0.2

Leather

Wood

0.3 - 0.4

-

Leather

Metal(Clean)

0.6

0.2

Leather

Metal(Wet)

0.4

-

Leather

Oak (Parallel grain)

0.61

-

Magnesium

Magnesium

0.6

0.08

Nickel

Nickel

0.7-1.1

0.28

Nylon

Nylon

0.15 - 0.25

-

Oak

Oak (parallel grain)

0.62

-

Oak

Oak (cross grain)

0.54

-

Platinum

Platinum

1.2

0.8

Plexiglas

Plexiglas

0.8

0..4-0.5

 Plexiglas

Steel

0.4 - 0.5

0.5

Polystyrene

Polystyrene

0.5

0.3-0.35

Polystyrene

Steel

0.3-0.35

0.2

Rubber

Rubber

1.16

-

Saphire

Saphire

0.2

0.2

Silver

Silver

1.4

0.55

Sintered Bronze

Steel

-

0.13

Solids

Rubber

1.0 - 4.0

-

Steel

Aluminium Bros

0.45

-

Steel

Brass

0.35

0.19

Steel(Mild)

Brass

0.51

-

Steel (Mild)

Cast Iron

-
0.183

Steel

Cast Iron

0.4

-

Steel

Copper Lead Alloy

0.22

0.145

Steel (Hard)

Graphite

0.21

-

Steel

Graphite

0.1

-

Steel (Mild)

Lead

0.95

0.3

Steel (Mild)

Phos. Bros

-

0.173

Steel

Phos Bros

0.35

-

Steel(Hard)

Polythened

0.2

-

Steel(Hard)

Polystyrene

0.3-0.35

-

Steel (Mild)

Steel (Mild)

0.74

0.09-0.19

Steel(Hard)

Steel (Hard)

0.78

0.029-0.12

Steel

Zinc (Plated on steel)

0.5

-

Teflon

Steel

0.04

0.04

Teflon

Teflon

0.04

0.04

Titanium Alloy Ti-6Al-4V(Grade 5)

Aluminium Alloy 6061-T6

0.41

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Titanium Alloy Ti-6Al-4V(Grade 5)

0.36

-

Titanium Alloy Ti-6Al-4V(Grade 5)

Bronze

0.36

-

Tungsten Carbide

Tungsten Carbide

0.2-0.25

0.12

Tungsten Carbide

Steel

0.4 - 0.6

0.08-0.2

Tungsten Carbide

Copper

0.35

-

Tungsten Carbide

Iron

0.8

-

Wood

Wood(clean)

0.25 - 0.5

-

Wood

Wood (Wet)

0.2

-

Wood

Metals(Clean)

0.2-0.6

-

Wood

Metals (Wet)

0.2

-

Wood

Brick

0.6

-

Wood

Concrete

0.62

-

Zinc

Zinc

0.6

0.04

Zinc 

Cast Iron

0.85

-

table extracted from internet archive

Frequently Asked Questions on Static Friction Coefficient

1. What’s the coefficient of static friction of mild steel on mild steel for dry surfaces?

Ans: 0.74 (refer to the table above)

2. What’s the coefficient of static friction of wood on wood?

Ans: 0.25 – 0.5 (refer to the table)

3. What is the coefficient of friction between hard rubber?

Ans: 1.16 (refer to the table)

4. What is the coefficient of static friction between road and tires?

Ans: The coefficient of friction between road and tires largely depends on the threaded nature of the tire and the road surface condition.

For slick/racing tires, the friction coefficient can attain a very high value of 0.9 for a dry-smooth surface but can fall so low as 0.1 if the surface is wet.

For threaded/commercial tires, the coefficient of friction is about 0.7 for dry surfaces and 0.4 for wet surfaces

Now…Over to you

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was it helpful in any way and would you like to see more content like this?

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The 5 Laws of Friction (Fully Explained) – Afrilcate

The 5 Laws of Friction (Fully Explained) – Afrilcate

The 5 Laws of Friction (with Detailed Explanation)

by Stanley Udegbunam

updated Oct 13, 2020

laws of frictionn

Laws of Friction

  1. The maximum static frictional force (limiting friction) is directly proportional to the normal force. 
  2. Friction acts in a direction opposite to the direction of motion.
  3. The frictional force is independent of the area of contact surfaces.
  4. Frictional force depends on the nature of the surfaces in contact.
  5. Kinetic friction is independent of the sliding velocity.

RECAP

Overview of Friction

Friction is the resistance between contacting bodies when one moves relative to another.

It’s applicable in every sphere of life and this makes it one of the fundamental topics in physics.

There are three major types of friction.

  1. Static friction: friction force between an object at rest relative to a surface.
  2. Kinetic friction: friction force between surfaces that moves or slides relative to each other.
  3. Rolling friction: friction force that resists the rolling motion of an object against a surface.

Analyse

Detailed Explanation of Friction Laws

  1. The maximum static frictional force (limiting friction) is directly proportional to the normal force.

Fstatic α N

The maximum static frictional force is also known as the limiting friction.

This law simply tells us that as the normal reaction increases, the frictional force will also increase.

The normal reaction depends on the weight of the object.

This law of friction is known as the Amontons’  First Law.

1st law of friction
  1. Friction acts in a direction opposite to the direction of motion.

This law correlates with the definition of friction.

Since friction is a resisting force it will always act opposite to the direction of push or pull.

2nd law of friction
  1. The frictional force is independent of the area of contact surfaces.

consider these two Lego bricks of the same weight sitting on a wooden table.

  1. The frictional force is independent of the area of contact surfaces.

consider these two Lego bricks of the same weight sitting on a wooden table.

3rd Friction Law

One would think that block B will have a higher friction force because of its larger surface contact but that’s not true.

Do you know why?

Recall that pressure equals force divided by area of contact. P = F/A

For objects with the same mass, it means that the increase in the contact area is offset by a corresponding reduction in pressure.

The frictional force will therefore remain unchanged irrespective of the contact area of the object.

 

  1. Frictional force depends on the nature of the surfaces in contact.

The nature of the surfaces in contact affects the coefficient of friction.

Some surfaces like higher friction coefficient like rubber on pavements.

while others have a lower friction coefficient just like ice on steel.

Keep in mind that the coefficient of friction is the constant of proportionality in the friction formula.

Fstatic α N

Fstatic = µfrictionN

Where µ = coefficient of friction

The surface nature varies based on certain factors like:

  • specific material property,
  • smoothness or roughness of contact surface,
  • dryness or wetness of contact surface.

This particular friction law is also known as the Amontons’ second Law.

  1. Kinetic friction is independent of the sliding velocity.

Also known as the Coulomb’s Law of Friction, it says that the kinetic friction exerted between the contact surface of two dry solids is not affected by the magnitude of the relative surface velocity.

Kinetic friction depends on factors like:

  • the kinetic coefficient of friction and
  • normal force

It pays no attention to the relative speed between the contact area.

Over to You…

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Have you come across any conflicting or contradicting statements in the past?

Share your thoughts and answers with us in the comment section below.

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