U3, D2 ~ What is Friction?

​What is Friction?

​Why do we strike a match stick to light it up? The answer is friction. When you strike the match stick against the rough surface of the matchbox, the friction between the two surfaces generates heat. This heat causes the chemicals on the match head to react and ignite, producing a flame.

​What 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 shown below.

​What is Friction?

Watch the video below to learn more about friction and consider what our life would be like without friction:

Multiple Choice

What does kinetic friction do when sliding objects?

• a. Speeds things up

• b. Slows things down

• c. Has no effect on movement

• d. Makes things warmer

Multiple Choice

Why is friction important when walking?

• a. It helps you run faster

• b. It prevents slipping

• c. It allows you to jump higher

• d. It makes your shoes wear out

​What is Friction?

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.

​What is Friction?

Factors That Affect Friction

Friction is affected by the roughness of surfaces and the strength of the force pushing them together. 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. Friction can be reduced by making the surfaces smoother; for example, if the surfaces are rough wood, then sanding the surfaces can make them smoother. Using an oily or greasy substance called a lubricant between surfaces can reduce friction as it creates a smoother layer between the surfaces

Multiple Choice

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

• Heavier objects press together with greater force, and this causes greater friction between them. 

• Heavier objects press together with less force, and this causes less friction between them.

​What is Friction?

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. The heat from friction can be useful. It not only warms your hands, but it can also light up matches. 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.

Multiple Choice

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

• Friction is increased by adding oil to the engine.
• Oil causes the moving parts to stick together.
• Friction is eliminated by using water instead of oil.
• Oil reduces friction between moving parts in a car engine.

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.