# Friction

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Figure 1. The thermal energy generated during friction braking can be seen in this friction brake/rotor set-up during stress testing.[1]

Friction is an everyday force that is created by two surfaces interacting. When these surfaces slide against each other, this interaction increases the thermal energy of the two surfaces (the temperature goes up). While it is easy to think of friction as a 'bad' thing, friction is needed in order to drive (it's what pushes the wheels on our car forward and allows us to stop and turn), or even walk.

Friction is why:

Friction in engines and machines contributes to energy loss, which is what wears out the parts in a car (hence the need for lubricating oil). Friction is a non-conservative force, meaning energy is transferred to new forms not useful to the system (but doesn't disappear from the universe, see conservation of energy).

There are three types of friction:

1. Static friction, when surfaces do not move with respect to each other, like shoes on a floor while walking, or tires on the road while driving.
2. Kinetic friction, when surfaces move with respect to each other like shoes on a banana peel, or skidding tires on a road.
3. Rolling friction, which is also known as rolling resistance.

For a more complete description of physics please consult hyperphysics.

## Phet Simulation

The University of Colorado has graciously allowed us to use the following Phet simulation. This simulation explores the relationship between macroscopic motion, frictional forces, microscopic kinetic energy and temperature: