How friction affects movement

In a nutshell

Friction is a force that slows down the speed of a moving object on a surface. It is a force acting between two surfaces that are sliding against each other or, if the object is not moving, two surfaces that are trying to slide against each other. 

How it works

The direction of the force is always in the opposite direction to that in which the object is moving or trying to move in. As a result, friction will always slow down a moving object or keep an object from moving if it's trying to start moving. 

If an object is sliding against an inclined slope, the friction will act in the opposite direction to its movement.

​​1. Friction

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Energy transfer

When friction acts between two surfaces, some of the kinetic energy is transformed into thermal energy. This means that although friction can be useful, it also produces heat. Therefore friction wastes some of the energy and limits your maximum speed.  

Example

The brakes on a car work because applying the brakes creates friction between the pads and the discs inside of the car. This friction slows down the motion of the wheels and causes the car to stop moving.