Key words: Sliding friction, resistive force, static, kinetic, coeffiecint, nornal force, Physics, Ron Kurtus, School for Champions. Copyright © Restrictions

# CHK - Sliding Friction

by Ron Kurtus (revised 23 January 2015)

When you try to move a solid object along the surface of another solid object, the resistance to motion is called sliding friction.

If the resistion to sliding is greater than the force pushing the object, such that it does not move, the resistance is called static friction. Once the object is sliding, the resistance is call kinetic friction.

Although the friction equation holds for both situations, the coefficient of friction in the static case is larger than that in the kinetic case.

In the case of sliding hard objects, the area of the surfaces in contact do not affect the amount of friction.

Also, the velocity of sliding and the types of surfaces affect the coefficient of friction.

Questions you may have include:

• What is static sliding friction?
• What kinetic sliding friction?
• What are factors in coefficient of friction for sliding objects?

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

## Static sliding friction

When a force pushing on an object is not strong enough to overcome the resistance from friction and slide the object over a surface, the resistive force is called static friction. The friction equation holds for this situation:

FSr = μSN

where:

• FSr is the static resistive force of friction in pounds (lbs) or newtons (N)
• μS is the static coefficient of friction for the two surfaces (Greek letter "mu")
• N is the normal or perpendicular force pushing the two objects together in units of lbs or N
• μSN is μS times N

μS is a number greater than 0 (zero) and up to (infinity). When μS = ∞, the objects are stuck together and will not move.

## Kinetic sliding friction

If the force pushing on the object is sufficient to cause it to slide, the resistive friction force is called kinetic friction. The kinetic sliding friction equation is:

FKr = μKN

where:

• FKr is the kinetic sliding resistive force of friction
• μK is the kinetic coefficient of friction for the two surfaces (Greek letter "mu")

μK is a number between 0 (zero) and a number less than (infinity).

## Different coefficents of friction

Although the friction equation holds for both situations, the coefficients of friction are different. The static coefficient of friction is greater than the kinetic coefficient.

For example, the static coefficient of friction (μS) for Aluminum sliding on Steel is 0.61, while μK for the combination is 0.47.

### Velocity

In most cases, the velocity of the sliding object does not affect the coefficient of friction. However, at higher velocities, μK can change. There have not been many experiments to measure the change in μK with respect to the sliding velocity.

### Surfaces

Typically, you consider a hard, dry surface sliding on another hard, dry surface. However, there are several other possible situations.

• One surface hard and the other soft
• Both surfaces soft
• Sliding on a fluid (lubrication)

The type of surface affects the sliding friction.

## Summary

Sliding an object along the surface of another object results in sliding friction. When the resistion to sliding is greater than the force pushing the object, it is called static friction. Once the object is sliding, the resistance is call kinetic friction.

The friction equation holds for both situation. However, the static coefficient of friction is larger than the kinetic coeffecient. Also, the velocity of sliding and the types of surfaces affect the coefficient of friction.

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## Resources and references

Ron Kurtus' Credentials

### Websites

Friction Concepts - HyperPhysics

Friction - Wolfram Research Science World

Friction Resources - Extensive list

### Books

The following books are available from Amazon.com.

#### Basic

Complete Idiot's Guide To Physics by Johnnie T. Dennis; Alpha (2003) \$18.95

What Is Friction? (Ages 4-8) by Lisa Trumbauer; Children's Press (CT) (2004) \$4.95

Friction Science and Technology (Mechanical Engineering Series) by Peter J. Blau; Marcel Dekker Pub. (1995) \$89.95

Physics of Sliding Friction (NATO Science Series E:) by B.N. Persson, E. Tosatti; Springer Pub. (1996) \$358.00

## 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.

## Students and researchers

www.school-for-champions.com/science/friction_sliding.htm.

Please include it as a link on your website or as a reference in your report, document, or thesis.

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