Friction:

Physics the angle of a plane to the horizontal when a body placed on the plane will start to slide. The tangent of the angle of friction is coefficient of the static friction. The angle between this resultant and the normal reaction is called as angle of friction. The tangent of angle of friction is equal to coefficient of friction. There are occasions when it is suitable to use the resultant contact force.

When the 2 objects are in rough contact and friction is limiting, two contact forces act on each object; one is the normal reaction and the other is the frictional force. Friction is the force resisting the relative lateral motion of the solid surfaces, fluid layers, or material elements in contact. It is usually subdivided into a number of varieties:

Dry friction resists relative lateral motion of 2 solid surfaces in contact. Dry friction is subdivided into static friction between non-moving surfaces, and the kinetic friction. Lubricated friction or fluid friction resists relative lateral motion of 2 solid surfaces separated by a layer of gas or liquid.

Fluid friction is also used to describe the friction between layers within the fluid which are moving relative to each other.

Skin friction is a component of drag, force resisting the motion of solid body through a fluid.

Internal friction is the force resisting motion in between the elements making up a solid material while it undergoes deformation.

Friction is not a fundamental force, as it is derived from electromagnetic force between charged particles, including electrons, atoms, protons, and molecules, and so cannot be calculated from first principles, but instead must be found empirically. When contacting surfaces move relative to each other, the friction among the 2 surfaces converts kinetic energy into thermal energy, or heat. Contrary to previous explanations, kinetic friction is now understood not to be caused by surface roughness but by chemical bonding between the surfaces. Surface roughness and contact area, but, do affect the kinetic friction for micro- and nano-scale objects where surface area forces dominate inertial forces.