# Friction

Written by Jerry Ratzlaff on . Posted in Fluid Dynamics

## Friction

Friction ( $$FRICT$$ ) is the mechanical resistance to the relative movement of two surfaces.  The frictional force on each body is in an opposite direcrion to the motion of the other body.  Since no surface is perfectly smooth, all having at least a minute roughness, there will always be friction, no matter how small the friction.

### Types of Friction

• Fluid friction
• Kinetic friction (sliding friction)
• Rolling friction
• Static friction

### Friction formula

$$f =\mu F_n$$          $$friction \;=\; friction \; coefficient \;\;x\;\; normal \; force$$

Where:

$$f$$ = friction

$$\mu$$ (Greek symbol mu) = friction coefficient

$$F_n$$ = normal force

## Dynamic Friction

Dynamic friction (types are fluid, kinetic, and rolling friction) is resistance to relative movement of two bodies that are already in motion and rubing togeather.

## Fluid Friction

Fluid friction, which includes gasses and liquids, acts on objects when they are moving through fluid.  The force exerted on an object depends on the material, shape of, speed, and viscosity of the liquids.  As an example, flying through the air where you have to overcome the particles of liquid that is held in the air, also called drag force

## Friction Coefficient

Friction coefficient ( $$\mu$$ ) (also called Coefficient of Friction) is the ratio between two contacting surfaces and the frictional force that resists the normal force of the object.

### Friction Coefficient formula

$$\mu = \frac {F_a}{F_n}$$          $$friction \; coefficient \;=\; \frac { applied \; force } { normal \; force }$$

Where:

$$\mu$$ (Greek symbol mu) = friction coefficient

$$F_a$$ = applied force

$$F_n$$ = normal force

## Friction Coefficient of materials table

Extreme care is needed in using friction coefficients and additional independent references should be used.   For any specific application the ideal method of determining the coefficient of friction is by trials.

Material 1Material 2Coefficient of Friction
DryGreasy
StaticSlidingStaticSliding
Aluminum Aluminum 1.05 - 1.35 1.4 0.3 -
Aluminum Mild Steel 0.4 - - -
Brake Material Cast Iron - - - -
Brake Material Cast Iron (wet) 0.2 - - -
Brass Cast Iron - 0.3 - -
Brick Wood 0.6 - - -
Bronze Cast Iron - 0.22 - -
Bronze Steel - - 0.16 -
Cadmium Mild Steel - 0.46 - -
Cast Iron Cast Iron 1.1 0.15 - 0.07
Cast Iron Oak - 0.49 - 0.075
Chromium Chromium 0.41 - 0.34 -
Copper Cast Iron 1.05 0.29 - -
Copper Copper 1.0 - 0.08 -
Copper Mild Steel 0.53 0.36 - 0.18
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 0.4 0.1 - 0.6 0.09 - 0.12
Glass Metal 0.5 - 0.7 - 0.2 - 0.3 -
Glass Nickel 0.78 0.56 - -
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 -
Lead Cast Iron - 0.43 - -
Leather Wood 0.3 - 0.4 - - -
Leather Metal (clean) 0.6 - 0.2 -
Leather Metal (wet) 0.4 - - -
Leather Oak (parallel grain) 0.61 0.52 - -
Magnesium Magnesium 0.6 - 0.08 -
Nickel Nickel 0.7-1.1 0.53 0.28 0.12
Nickel Mild Steel - 0.64 - 0.178
Nylon Nylon 0.15 - 0.25 - - -
Oak Oak (parallel grain) 0.62 0.48 - -
Oak Oak (cross grain) 0.54 0.32 - 0.072
Platinum Platinum 1.2 - 0.25 -
Plexiglas Plexiglas 0.8 - 0.8 -
Plexiglas Steel 0.4 - 0.5 - 0.4 - 0.5 -
Polystyrene Polystyrene 0.5 - 0.5 -
Polystyrene Steel 0.3 - 0.35 - 0.3 - 0.35 -
Polythene Steel 0.2 - 0.2 -
Rubber Asphalt (dry) - 0.5 - 0.8 - -
Rubber Asphalt (wet) - 0.25 - 0.0.75 - -
Rubber Concrete (dry) - 0.6 - 0.85 - -
Rubber Concrete (wet) - 0.45 - 0.75 - -
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 0.44 - -
Steel (mild) Cast Iron - 0.23 0.183 0.133
Steel Cast Iron 0.4 - 0.21 -
Steel Copper Lead Alloy 0.22 - 0.16 0.145
Steel (hard) Graphite 0.21 - 0.09 -
Steel Graphite 0.1 - 0.1 -
Steel (mild) Lead 0.95 0.95 0.5 0.3
Steel (mild) Phos. Bros - 0.34 - 0.173
Steel Phos. Bros 0.35 - - -
Steel (hard) Polythened 0.2 - 0.2 -
Steel (hard) Polystyrene 0.3-0.35 - 0.3 - 0.35 -
Steel (mild) Steel (mild) 0.74 0.57 - 0.09 - 0.19
Steel (hard) Steel (hard) 0.78 0.42 0.05 - 0.11 0.029 - .12
Steel Zinc (plated on steel) 0.5 0.45 - -
Teflon Steel 0.04 - 0.04 0.04
Teflon Teflon 0.04 - 0.04 0.04
Tin Cast Iron - .32 - -
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 0.21 - -

## Friction Factor

Also known as the Moody Friction Factor or Darcy Weibach friction factor it is a dimensionless number used in internal flow calculations with the Darcy-Weisbach equation. Depending on the Reynolds Number, the friction factor may be calculated one of several ways.

## Kinetic Friction

Kinetic friction ( $$f_k$$ ) (also called dynamic friction or sliding friction) is the force opposing two objects rubbing together that are moving relative to each other.

### Kinetic Friction formula

$$f_k = \mu_k F_n$$          $$kinetic \; friction \;=\; kinetic \; friction \; coefficient \;\;x\;\; normal \; force$$

Where:

$$f_k$$ = kinetic friction

$$\mu_k$$ (Greek symbol mu) = kinetic friction coefficient

$$F_n$$ = normal force

Solve for:

$$\mu _k = \frac {f_k}{F_n}$$

$$F_n = \frac {f_k}{\mu_k}$$

## Kinetic Friction Coefficient

Kinetic friction coefficient ( $$\mu_k$$ ) (dimensionless number) (also called coefficient of kinetic friction) is the amount of force that resists motion at a constant velocity

### Kinetic Friction Coefficient FORMULA

$$\mu_k = \frac {f_k}{F_n}$$          $$kinetic \; friction \; coefficient \;=\; \frac { kinetic \; friction } { normal \; force}$$

Where:

$$\mu_k$$ (Greek symbol mu) = kinetic friction coefficient

$$f_k$$ = kinetic friction

$$F_n$$ = normal force

## Rolling Friction

Rolling friction is weaker than kinetic (sliding) or static friction, this one acts on objects when they are rolling over a surface.

## Sliding Friction

Sliding friction (also called dynamic friction or kinetic friction) is resistance to relative movement of two bodies that are already in motion and rubing togeather.

## Static Friction

Static friction ( $$f_s$$ ) is the force that resists relative movement and keeps objects at rest.  Static friction happens between zero and the smallest force needed to start the motion of an object.  In order for an object to move the static friction must be overcome, when this happens you experience kinetic friction.

### Static Friction formula

$$f_s = \mu_s F_n$$          $$static \; friction \;=\; static \; friction \; coefficient \;\;x\;\; normal \; force$$

Where:

$$f_s$$ = static friction

$$\mu _s$$ (Greek symbol mu) = static friction coefficient

$$F_n$$ = normal force

Solve for:

$$\mu _s = \frac {f_s}{F_n}$$

$$F_n = \frac {f_s}{\mu_s}$$

## Static Friction Coefficient

Static friction coefficient ( $$\mu_s$$ ) (dimensionless number) (also called coefficient of static friction) is the amount of force that resists motion that is on the verge of motion.

### Static Friction Coefficient formula

$$\mu_s = \frac {f_s}{F_n}$$          $$static \; friction \; coefficient \;=\; \frac { static \; friction } { normal \; force }$$

Where:

$$\mu_s$$ (Greek symbol mu) = static friction coefficient

$$f_s$$ = static friction

$$F_n$$ = normal force