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Normal Force in Rolling Friction

by Ron Kurtus (15 November 2016)

The normal force in rolling friction is the perpendicular force pushing the wheel to the surface on which it is rolling. It is an essential part of the standard rolling friction equation.

Situations include a wheel rolling on a plane surface, a wheel with an axle on a plane surface, and a wheel acting as a roller between two objects.

Questions you may have include:

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



Standard rolling friction equation

The standard friction equation for a wheel rolling on another surface is:

Fr = μrN

where:

Normal force for a wheel on an object

When a wheel is rolling on the level surface of another object, the normal force is typically the weight of the wheel.

Normal force is weight of the wheel

Normal force is weight of the wheel

On incline or curved surface

When the wheel is on an incline or curved surface, the normal force varies with the cosine of the angle between the weight and the direction perpendicular to the surface. Thus,

N = W*cos(α)

where

(See Friction on an Inclined Surface for more information.)

The normal force varies with the angle of the incline

The normal force varies with the angle of the incline

Normal force for a wheel with an axle

The normal force of a wheel that has an axle held in some other object—such as a cart or other vehicle—is the weight or the wheel plus the weight of the cart.

N = W1 + W2

where

Normal force is sum of wheel and object weights

Normal force is sum of wheel and object weights

Normal force for rollers between objects

When wheels are placed between two objects, they act as rollers. There is rolling friction both on the top and bottom of the wheel. The normal force on the top is the weight of the top object, and the normal force on the bottom is the weight of the top object and the wheel.

Normal forces for roller configuration

Normal forces for roller configuration

Thus, the friction between the top object and the rollers is Fr1 = μr(W1), where W1 is the weight of the upper object and the normal force on each roller. Fr1 is the friction force acting on the rollers.

The friction of each roller on the bottom object is Fr2 = μr(W1 + W2), where W2 is the weight of the roller and W1 + W2 is normal force on the bottom object.

Fr2 is the friction force that acts along with Fr1 to cause the rollers to turn.

Summary

When a wheel is rolling on a plane surface, the normal force on the surface is the weight of the wheel, along with a cosine factor is the surface is on an incline. When a wheel with an axle is rolling on a plane surface, the normal force on that surface is the weight of the cart plus that of the wheel. In the case of rollers between two objects, the weight or the top object and the roller create the normal force.


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Resources and references

Ron Kurtus' Credentials

Websites

Friction Resources - Extensive list

Rolling friction and rolling resistance - includes coefficients - Engineering Toolbox

Rolling Friction - simple explanation - Davidson College

Books

Top-rated books on Friction Science

Top-rated books on Friction Experiments


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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