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# 4.12: Friction

Difficulty Level: At Grade Created by: CK-12
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Did you ever rub your hands together to warm them up, like the young man in the opening image? Why does this make your hands warmer? The answer is friction.

### What Is Friction?

Friction is a force that opposes motion between any surfaces that are touching. Friction can work for or against us. For example, putting sand on an icy sidewalk increases friction so you are less likely to slip. On the other hand, too much friction between moving parts in a car engine can cause the parts to wear out. Other examples of friction are illustrated in the two Figures below and below

These photos show two ways that friction is useful

These photos show two ways that friction can cause problems

### Why Friction Occurs

Friction occurs because no surface is perfectly smooth. Even surfaces that look smooth to the unaided eye make look rough or bumpy when viewed under a microscope. Look at the metal surfaces in the Figure below. The aluminum foil is so smooth that it’s shiny. However, when highly magnified, the surface of metal appears to be very bumpy. All those mountains and valleys catch and grab the mountains and valleys of any other surface that contacts the metal. This creates friction.

### Factors That Affect Friction

Rougher surfaces have more friction between them than smoother surfaces. That’s why we put sand on icy sidewalks and roads. You can’t slide as far across ice with shoes as you can on the blades of skates (see Figure below). The rougher surface of the soles of the shoes causes more friction and slows you down.

Q: Heavier objects also have more friction. Can you explain why?

A: Heavier objects press together with greater force, and this causes greater friction between them. Did you ever try to furniture across the floor? It’s harder to overcome friction between a heavier piece of furniture and the floor than between lighter pieces and the floor.

### Friction Produces Heat

You know that friction produces heat. That’s why rubbing your hands together makes them warmer. But do you know why? Friction causes the molecules on rubbing surfaces to move faster, so they have more energy. This gives them a higher temperature, and they feel warmer. Heat from friction can be useful. It not only warms your hands. It also lets you light a match as shown in the Figure below. On the other hand, heat from friction between moving parts inside a car engine can be a big problem. It can cause the car to overheat.

Q: How is friction reduced between the moving parts inside a car engine?

A: To reduce friction, oil is added to the engine. The oil coats the surfaces of the moving parts and makes them slippery. They slide over each other more easily, so there is less friction.

### Summary

• Friction is a force that opposes motion between any surfaces that are touching.
• Friction occurs because no surface is perfectly smooth.
• Rougher surfaces have more friction between them. Heavier objects also have more friction because they press together with greater force.
• Friction produces heat because it causes the molecules on rubbing surfaces to move faster and have more energy.

### Review

1. Define friction, and explain why it occurs.
2. Identify two factors that affect friction.
3. Why does friction warm your hands when you rub them together?
4. Outside wooden steps may get slippery when they are wet. How could you make them less slippery?

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### Vocabulary Language: English

TermDefinition
friction Force that opposes motion between two surfaces that are touching.

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