READ: Friction

READ: Friction

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. 

These photos show two ways that friction is useful:

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. Increasing the area of surfaces that are touching also increases the friction between them. That’s why you can’t slide as far across ice with shoes as you can on the thin blades of skates (see figure below). The greater surface area of the soles of the shoes cause more friction and slow you down.

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.