The contact force that resists the motion of one surface across another surface is called friction.

There are two main types of friction:

* Static friction: This force acts on an object at rest and prevents it from moving. It is the force you need to overcome to get the object moving.

* Kinetic friction: This force acts on an object that is already moving, and it opposes the motion.

Friction is caused by the microscopic irregularities on the surfaces of the objects in contact. These irregularities interlock and resist the sliding motion.