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Explanation of the factors involved in decreasing or increasing the amount of sliding friction between hard surfaces by Ron Kurtus - Succeed in Understanding Physics. Key words: Physical Science, coefficient of friction, normal force, lubrication, rolling friction, molecular forces, roller bearings, different materials, School for Champions. Copyright © Restrictions

# Changing Sliding Friction on Hard Surfaces

by Ron Kurtus (16 February 2008)

The factors that determine the amount of sliding friction between two hard surfaces are the coefficient of friction and the normal force pushing the surfaces together. By "hard surfaces" we mean those that are deformed only a negligible amount when pushed together. You can increase or decrease the amount of resistive force of sliding friction by changing the coefficient of sliding friction, by replacing it with another type of friction or by changing the normal force.

Questions you may have include:

• How do you change the coefficient of friction?
• How do you replace the type of friction?
• How can you change the normal force?

This lesson will answer those questions.

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## Changing coefficient of friction

One way to increase or decrease the force of friction from sliding two hard surfaces together is to change the coefficient of friction between the surfaces. The coefficient of friction is a number between 0 (zero) and ∞ (infinity) and is a result of the molecular adherence between the two materials. The relationship between the surface roughness of the materials also a factor, although small.

### Standard friction equation

The relationship between friction and its coefficient can be seen from the standard friction equation

Fr = μN

where:

• Fr is the resistive force of friction
• μ is the coefficient of friction for the two surfaces (Greek letter "mu")
• N is the normal or perpendicular force pushing the two objects together
• μN is μ times N

Note that with the standard sliding friction between hard surfaces, the area of the surfaces is not a factor. In other words, large areas sliding against each other will have the same friction as small areas, provided the coefficients and normal forces are the same.

### Changing roughness

One method to increase the coefficient of friction is to make the surfaces rough or less smooth. A shoe sole that contains a pattern of grooves or ripples will provide much more friction when walking on slippery ice than would a worn-out, smooth sole.

Polishing the surfaces that come into contact will reduce the coefficient of friction between the surfaces and thus reduce the amount of friction encountered. But note that if you polish the surfaces to be extremely smooth and flat, molecular effects may take place.

### Molecular effects

When you slide two extremely flat and highly polished metal surfaces together, molecular attraction adds to the coefficient of friction. In other words, the coefficient decreases as you reduce roughness up to a certain point where it then increases due to the molecular effects.

#### Area now a factor

Once molecular effects come into play, the area of the surfaces in contact are a factor in the friction. Thus, the greater the area, the greater the friction caused by molecular effects.

#### Sticky materials

Some materials have a high coefficient of friction, not because their surface is rough, but because the material is "sticky" and molecular attraction adds to the common coefficient of friction. Rubber is a good example of molecular attraction adding to the standard coefficient of friction caused by surface roughness.

### Different materials

Another method to increase friction by increasing the coefficient is by using different materials that have a higher value for the coefficient of friction. Materials used in automobile brake pads do a much better job today than the materials used in the early days of automobiles.

Using material combinations that have a lower coefficient of friction between them is another way to reduce friction.

#### Kinetic Friction

Steel on steel (dry)

0.6

0.4

Teflon on steel

0.041

0.04

Coating a material with Teflon will greatly reduce the friction

## Replacing type of friction

Although reducing the coefficient of friction is a method to decrease the force of friction, it has its limitations. For example, you can only polish a material so much until the coefficient actually increases due to molecular effects. A better way to reduce the coefficient of sliding friction is to eliminate it altogether by replacing it with rolling friction and/or fluid friction.

### Rolling friction

Instead of sliding surfaces together, you can add rollers between the surfaces, thus changing over to rolling friction.

When making the Great Pyramids, the ancient Egyptians had great difficulty sliding the huge granite slabs to the pyramid site. So they reduced the friction by rolling the slabs on logs.

### Fluid friction

By adding a thin layer of oil or even water between two objects, you convert the sliding friction to fluid friction. This lubrication is used in many applications to reduce wear and to make the sliding much easier.

#### Kinetic Friction

Steel on steel (dry)

0.6

0.4

Steel on steel (greasy)

0.1

0.05

Adding grease as a lubricant reduces the coefficient of friction

#### Sometimes increases friction

In some situations, adding a lubricant actually increases the friction, due to molecular effects. For example, a coin will slide down a plastic ramp. But if you wet the ramp, the coin will stick in one place. You might think the water would be a lubricant, but for small objects, its molecular force is stronger than the gravitational pull on the object.

Another example is trying to use a heavy, thick oil or grease as a lubricant. The friction may be much great than if you used a light oil or even no lubricant at all.

### Combining rolling and fluid frictions

The axles in a bicycle, automobile and other devices used to rotate in a hub, sliding metal against metal. Soon a lubricant was added to reduce the friction. But then a set of ball bearings was inserted between the axle and hub to change the friction to rolling friction. The ball bearings, along with lubricating oil, greatly reduce the friction in rotating a wheel.

Ball bearings reduce axle friction

## Changing normal force

The normal force, N, is the perpendicular force pushing the surfaces together. By increasing N, you increase the resistive force of friction. Decreasing N, lowers the friction.

Note that changing friction by changing the normal force also applies to situations where you have included lubrication or rolling friction.

The force clamping automobile brake pads to the brake disk determines the friction used to slow down the car. Of course, releasing the brakes causes N to become zero, resulting in no friction force.

When you step on the brakes in your car, the brake calipers clamp onto metal disks that are part of your disc brake system. The friction between the caliper and the disk is sufficient to slow down and stop your wheels. That is where the tires take over. Their friction on the road then stops your car from moving.

If the normal force is the weight of objects you are sliding across the floor, adding or subtracting weight will increase or decrease the sliding friction. But also, if you are pulling an object with a rope, pulling at an upward angle will reduce the normal force.

Reducing normal force by pulling at an upward angle

## Summary

Coefficient of friction and the normal force pushing the surfaces together are the factors that determine the amount of sliding friction between two hard surfaces. You can increase or decrease the amount of resistive force of sliding friction by changing the coefficient of sliding friction, by replacing it with another type of friction or by changing the normal force.

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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

Tribology: Friction and Wear of Engineering Materials by I. M. Hutchings; CRC Press (1992) \$68.88

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

## Questions and comments

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

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