# Friction: Definition and Types

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• 0:04 What Is Friction?
• 2:02 Types of Friction
• 3:27 Lesson Summary

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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

You experience friction all the time, and you should be glad you do! Friction helps keep stationary objects in place as well as slow moving objects down as they slide across a surface. This lesson identifies what friction is and explains the two ways we find this force on earth.

## What is Friction?

You may not realize it, but you are already quite familiar with friction. Have you tried to push a box across the floor only to encounter resistance against you? Ever get a nice burn on the back of your legs going down a metal slide? How about skidding to a stop on a wet or icy road after slamming on the brakes in your car? Friction was involved in all of these events!

Friction is the force exerted by the surface of an object when another object moves against it. Friction occurs in the direction opposite to motion, and because of this, it is a force that affects the motion of objects. When you push a box across the floor, friction works against the box in the direction opposite to the box's motion. When you go down a slide, friction works opposite to your downward movement. When you slam on the brakes and go sliding, friction works opposite to your direction of skid, helping to eventually stop your sliding altogether.

When two objects rub together, it sets off attractive forces between the molecules of the objects, causing friction. For this reason, friction can occur between just about anything in any phase of matter - solids, liquids, and gases. Friction occurs between two objects like the box and the floor, but it can also occur between fish and the water they swim in, and objects falling through the air. The friction due to air has a special name: air resistance.

Because friction results from attractive forces between the objects' surfaces, the amount of friction depends on the materials of those two interacting objects. Try skating across a smooth lake, and you'll find it much easier than skating across a rough, gravel road! The slide you go down? You'll likely experience some friction either way, but putting on long pants makes the trip much smoother than when the slide comes in contact with the skin on the back of your legs when you're wearing shorts.

## Two Types of Friction

Friction is a special force because there are two types to consider. The first type is sliding friction, which is what we have been talking about so far. Sliding friction occurs with an object's movement and resists that movement, slowing the object down. As the box is pushed across the floor, or you go down the slide, friction works between the surfaces, helping to slow or stop the movement that is occurring.

But there is also static friction, which occurs when the two objects are at rest. In this case, it prevents motion instead of slowing it. How does this work? Let's take that box you're trying to move again, but this time you haven't started pushing it yet. Once you do start pushing on it, in order to set it in motion, you have to overcome the static friction that keeps the box from moving. If the static friction from the floor is 15 N, you will have to push with a force greater than that to overcome the static friction and get the box sliding.

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