* Acts between two surfaces that are not moving relative to each other.

* Prevents objects from sliding or rolling.

* The force of static friction is proportional to the normal force between the surfaces.

Sliding friction

* Acts between two surfaces that are moving relative to each other.

* Resists the motion of objects sliding against each other.

* The force of sliding friction is proportional to the normal force between the surfaces and the coefficient of kinetic friction.

Rolling friction

* Acts between a surface and a rolling object.

* Resists the motion of objects rolling against a surface.

* The force of rolling friction is proportional to the normal force between the surfaces and the coefficient of rolling friction.

Fluid friction

* Acts between a surface and a fluid (liquid or gas) moving past it.

* Resists the motion of objects through fluids.

* The force of fluid friction is proportional to the viscosity of the fluid, the velocity of the object relative to the fluid, and the surface area of the object in contact with the fluid.