# What is Friction? - Definition, Formula & Forces

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• 0:00 Forces and Motion
• 0:50 Types of Friction
• 4:20 Measuring Coefficients…
• 5:25 Causes of Friction
• 5:54 Examples of Friction
• 8:14 Lesson Summary

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Lesson Transcript
Instructor: Elena Cox
Friction is the force that opposes the motion of an object. Learn about the sources of friction, identify examples of friction and test your knowledge with quiz questions.

## What is Friction?

To stop a moving object, a force must act in the opposite direction to the direction of motion. For instance, if you push your book across your desk, the book will move. The force of the push moves the book. As the book slides across the desk, it slows down and stops moving. The force that opposes the motion of an object is called friction.

## Forces and Motion

Look at this diagram. At first, the book is at rest. A push causes the book to slide across the desk. The force of the push (big F) keeps the book moving. As the book slides cross the desk, a force of friction (f) acts in the opposite direction. The friction slows down the motion of the book. Finally, the book is once again at rest.

## Types of Friction

There are different types of friction. A book moving across the desk is an example of sliding friction. As the book slides cross the desk, the bottom of the book is touching the desk. The source of the friction is the contact between the surface of the book and the desk. The weight of the object and the type of surface it moves over determine the amount of sliding friction present between the two objects. A heavy object exerts more pressure on the surface it slides over, so the sliding friction will be greater.

Air, water and oil are all fluids. Air resistance is a type of fluid friction. As an object falls, air resistance pushes up on the object.

When you ride a bicycle, the contact between the wheel and the road is an example of rolling friction. When an object rolls over a surface, the force needed to overcome rolling friction is much less than that needed to overcome sliding friction.

## Kinetic Friction

When you moved your book across the desk, the book experienced a type of friction that acts on moving objects. This force is known as kinetic friction force. It is exerted on one surface by another when the two surfaces rub against each other because one or both surfaces are moving. If you stack additional books on top of the first book to increase the normal force, the kinetic friction force will increase. Let's look at the formula for kinetic friction force.

There is a linear relationship between the kinetic friction force and the normal force. The coefficient of kinetic friction relates the friction force to the normal force. The kinetic friction force (F(f, kinetic)) equals the product of the coefficient of kinetic friction (µ(k)) and the normal force (F(N)). F(f, kinetic) = µ(k) * F(N)

## Static Friction

Imagine trying to push a couch across the floor. You push on it with a small force, but it does not move. This is because it is not accelerating. Newton's laws tell you that the net force on the couch must be zero. There must be a second horizontal force acting on the couch, one that opposes your force and is equal in size. This force is static friction force, which is the force exerted on the surface by another when there is no motion between the two surfaces.

Static friction force acts in response to a force trying to cause a stationary object to start moving. If there is no such force acting on an object, the static friction force is zero. If there is a force trying to cause motion, the static friction force will increase up to a maximum value before it is overcome and motion starts.

Now let's look at the formula for static friction force. The static friction force (F(f, static)) is less than or equal to the product of the coefficient of static friction (µ(s)) and the normal force (F(N)).

The maximum static friction force relates to the normal force in a similar way as the kinetic friction force. In the equation for maximum static friction force, µ(s) is the coefficient of static friction between two surfaces. The maximum static friction force that must be overcome before motion can begin is µ(s) * F(N). In the example of pushing the couch, the maximum static friction force balances the force of the person pushing on the couch the instant before the couch begins to move.

## Measuring Coefficients of Friction

On what does a friction force depend? The materials that the surfaces are made of play a role. For example, imagine trying to play basketball while wearing socks instead of athletic shoes. You would slip and slide all over the basketball court. Shoes help provide the forces necessary to quickly change directions while running up and down the court. There is more reaction between your shoes and concrete than there is between your socks and a polished wood floor.

This table shows coefficients of static friction (µ(s)) and coefficients of kinetic friction (µ(k)) between various surfaces. The coefficients of friction show how easily one object can slide against another. These coefficients are estimates for each combination of surfaces. Exact measurements of coefficients of friction are quite sensitive to the conditions of the surfaces and are determined experimentally.

Another important fact regarding the table is that all the measurements were made on dry surfaces (with exception of the oiled steel). Wet surfaces behave quite differently than dry surfaces.

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