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Friction Overview

by Ron Kurtus (revised 9 October 2016)

Friction is a force that resists the motion of an object or material that is in contact with another object or material. It is often called the resistive force of friction. It can cause problems but is also useful and necessary.

The resistance can result in static friction, where objects do not move with respect to each other. Or, it can be kinetic or dynamic, where there is motion.

The standard friction equation includes the coefficient of friction between materials.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Uses, problems, and causes

Friction has a variety of uses, problems, and causes.

Uses of friction

Since friction is a resistance force that slows down or prevents motion, it is necessary in many applications where you might want to hold items or do things and prevent slipping or sliding. In those cases, there is an advantage of having friction.

(See Uses of Friction for details)

Problems from friction

A number of problems can be caused by friction between objects or materials. Friction can be a nuisance, because it can hinder motion and cause the need for expending extra energy. Friction can also cause parts in contact to heat up and can cause them to wear out.

(See Problems from Friction for details)

Causes of friction

The causes of friction are molecular adhesion, surface roughness, and deformations.

Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Trying to slide objects against each other requires breaking these adhesive bonds.

Surface roughness is a factor when the materials are rough enough to cause serious abrasion. This is called the sandpaper effect.

When one or both of the materials is relatively soft, much of the resistance to movement is caused by deformations of the objects or by a plowing effect.

(See Causes of Friction for details)

Modes and types

Friction exists in several modes and three types.

Modes of friction

The resistive force of friction can occur in two different modes: static and kinetic. There is no motion in the static mode of friction, and there is relative motion between the objects or materials in the kinetic or dynamic mode of friction.

Between the static and kinetic modes, there is a transitional phase of friction, where the objects or materials change from static to moving with respect to each other.

(See Modes of Friction for details)

Types of friction

There are three major types of friction: sliding, rolling, and fluid friction. Each can be static (stationary) or kinetic (moving). Sliding and rolling friction concern moving an object along the surface of another object. With sliding and rolling friction, one or both objects may be hard or soft.

Fluid friction can concern lubrication between objects, as well as resistance between layers of fluids.

(See Types of Friction for details)

Friction equation

The standard friction equation shows the relationship between the resistive force of friction, the coefficient of friction and the normal force pushing the objects together:

Ff = μN

where:

μ is a number between 0 (zero) and (infinity).

(See Standard Friction Equation for details)

Summary

Friction is a force that resist the motion of the object that is in contact with another object or material. When the objects don't move, the friction is called static. When they do move, the friction is called kinetic. The different types friction are sliding, rolling and fluid friction. The cause of friction is a combination of molecular adhesion, surface roughness, and deformation effects. There is a general equation for friction.


Don't let the resistive forces of others slow you down


Resources and references

Ron Kurtus' Credentials

Websites

Friction Resources - Extensive list

Friction Concepts - HyperPhysics

RoyMech (UK) - Friction Factors

Books

Top-rated books on Friction Science

Top-rated books on Friction Experiments

Friction Science and Technology (Mechanical Engineering Series) by Peter J. Blau; Marcel Dekker Pub. (1995)

Control of Machines with Friction (The International Series in Engineering and Computer Science) by Brian Armstrong-Hélouvry; Springer Pub. (1991)


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


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