Key words: Friction, fluid, Physics, Physical Science, viscosity, texture, lubrication, turbulence, coefficient of friction, deformations, liquid. gas, water, oil, mud, air, surface, golf ball, dimples, Ron Kurtus, School for Champions. Copyright © Restrictions

# Changing Fluid Friction

by Ron Kurtus (18 October 2005)

Fluid friction occurs when a solid object travels through a liquid or gas. There is also resistance to motion within a fluid, but that is usually classified under fluid dynamics and is not covered here.

Factors that determine the amount of fluid friction on a solid object include the viscosity of the fluid, the surface texture of the solid object and the shape of the object. By changing any of these factors, you can increase or decrease the amount of resistive force of fluid friction.

Questions you may have include:

• How do you change friction caused by viscosity?
• How can you change friction caused by the object surface?
• How can you change friction caused by shape?

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

## Changing viscosity friction

Viscosity is the resistance to flow or changing the shape of a fluid. It is a factor in fluid friction.

### Material affects friction

Moving a solid object through air is much easier than moving through water because of the fact that water has a greater density and a higher viscosity. Likewise, it is easier to move an object through water than it is through a heavy oil.

If you can change the type of fluid that is resisting motion, you would obviously use a less dense fluid to reduce friction and a denser fluid to increase it.

### Effect of heat on viscosity

Increased heat will reduce the viscosity of a fluid and thus the fluid friction. A good example is how a gasoline engine may be difficult to start on a cold winter day, because its oil is so thick. Once the engine heats up, the oil becomes thinner and the engine easily operates.

Thus you can reduce fluid friction by heating the fluid.

### Lubrication

Fluids are often used in lubrication, because they reduce the friction of solid parts rubbing against each other. Thin oils produce less friction than thicker oils with more viscosity. In engines, there is a trade-off, because once a thin oil becomes very hot, it may become too thin to protect the engine. But also a thick oil may be too thick and have too much friction to even allow the engine to start in very cold weather. Multi-grade oils seem to be a good compromise to the heat and cold problem.

A thin layer of mud can be used to lubricate objects sliding together, but it the layer is too thick, the resistance to motion increases as the object must plow through the thick fluid.

## Changing surface characteristics

You can reduce the friction of a fluid sliding along a surface by improving the surface characteristics or texture of the solid object. Seldom are the surface characteristics changed to purposely increase the fluid friction.

### Rough surface

A rough surface texture will provide more resistance than a smooth surface in most cases. Even if the surface is smooth, having protrusions such as rivets and screw heads can increase the friction.

### Turbulence factor

An interesting aspect of fluid friction is that setting up tiny areas of turbulence on the surface will reduce the friction even more. A good example of that are the dimples on the surface of a golf ball. If an ball had a completely smooth surface, it would not fly as far as an identical ball with a dimpled surface. The reason is that each dimple creates a small area of turbulence on the surface. This means the air is then flowing over air in that area and not along the surface of the ball, thus reducing the resistive friction.

Dimples on golf ball reduce surface friction

But note that if the dimples were too large or deep, then the turbulence would cause the ball to slow down. Dimple dimensions and locations are very important in the design of golf balls.

## Changing object shape

The shape of the solid object moving through a fluid is seldom classified under friction. Usually it is a subject of fluid dynamics, air resistance or water resistance.

### Streamlining reduces friction

By streamlining the object moving through a fluid, you can greatly decrease the resistance. Moving a flat surface through a fluid such as air or water meets with much more resistance than does a sharply curved surface.

A shark is streamlined so it can move through water easily

### Increasing friction

When an airliner lands and is trying to slow down, large flaps on the wings provide a flat surface and extra air resistance to help the aircraft some to a stop.

Also, opening the windows in your car increases the air resistance due to the airflow inside the vehicle.

## Summary

Viscosity of the fluid, how smooth the surface of the solid object is, and the shape of the object are factors in changing fluid friction. By decreasing fluid friction you can make it easier to move objects through a fluid, while increasing friction if often used to slow down motion or to stop a moving object.

Be fluid in your ability to change

## 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

Friction and Lubrication in Mechanical Design (Mechanical Engineering Series) by Ali Seireg; Marcel Dekker Pub. (1998) \$199.95

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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