Friction is a force resulting from the contact of two surfaces. This force opposes motion.. Unlike gravity and electromagnetism, friction is not a conservative force. For solid bodies, one can distinguish two types of friction:
- Static friction
- the force that is needed for motion to start if the surfaces are initially at rest with respect to each other.
- Kinetic friction
- the force that is caused by friction once the surfaces are moving with respect to each other.
For solid surfaces, both types of friction depend (to a good approximation) only on the normal force. In particular, the dynamic friction does not depend on the velocity.