# Reducing Sliding Friction

by Ron Kurtus (revised 31 October 2016)

You can reduce the resistive force of sliding friction by applying lubrication between the two surfaces in contact, by using rollers, or by decreasing the normal force.

Questions you may have include:

• How does lubrication 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. Useful tool: Units Conversion

## Applying lubrication

You can reduce the sliding friction between two surfaces by applying a form of lubrication between the objects.

Lubrication can consist of some fluid or grease, where fluid friction—what has a much lower coefficient of friction— takes over.

### 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.

## 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.

### 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 reduce the resistive force of sliding friction by applying lubrication between the two surfaces in contact, by using rollers, or by decreasing the normal force.

## Resources and references

Ron Kurtus' Credentials

### Websites

Friction Resources - Extensive list

Friction Concepts - HyperPhysics

### Books

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_reducing.htm

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