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# Friction

## The resistance that one surface or object encounters when moving over another.

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What Causes Friction?

### What Causes Friction?

Credit: Robert Lopez
Source: CK-12 Foundation

Everyone knows that rubbing your hands together causes them to warm up, and many people know the heat comes from friction, but what exactly causes friction, and why does it create heat?

#### How It Works

Credit: F Delventhal
Source: http://www.flickr.com/photos/krossbow/339106135/

Going down a slide creates friction between the person and the slide [Figure2]

Friction is actually a bit of a mystery, even today!  We do know that some of the extra energy (that we feel as heat) generated between two rough surfaces comes from the force needed to break off tiny bits of the rough surfaces when they slide past each other.  We also know that there is a similar process between two smooth surfaces caused by the surface molecules attracting each other.  Even soft or squishy surfaces generate friction, as the surfaces bend around each other in tiny curves and get in the way of each other.

When surfaces resist sliding past each other, they move more slowly than they would otherwise.  Energy is equal to mass times velocity, and when the velocity (the speed of the surfaces sliding past each other) is reduced without changing the mass of the objects, some energy must be released to keep the balance.  The released energy is what we feel as heat!

#### Show What You Know

1. In general, which surfaces cause the most friction (also known as having a greater coefficient of friction) as they slide past each other?
1. Bronze and Iron
2. Glass and Glass
3. Leather and Oak
4. Rubber and Dry Asphalt
2. Do semi trucks have so many tires to improve traction?
3. Does force affect friction? How?
4. Why do road bicycles have such narrow tires, while mountain bikes (even racing versions) have wide, knobby tires?
5. What causes the rolling friction of the wheels on a train?

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