When two bodies are in contact, one upon the other and a force is applied to the upper body to make it move over the surface of the lower body, an opposing force is set up in the plane of the contract which resists the motion. This force is the force of friction or simply friction.
The force of friction always acts parallel to the surface of contact and opposite to the direction of motion.
When one body is at rest in contact with another, the friction is called Static Friction.
When one body is just on the point of sliding over the other, the friction is called Limiting Friction.
When one body is actually sliding over the other, the friction is called Dynamic Friction.
Coefficient of Friction (μ)
The ratio of limiting friction ‘F’ to the normal reaction ‘R’ acting between two surfaces in contact is called the coefficient of friction (μ).
μ = F / R
F = μ R
Stoke found that bodies moving through fluids (liquids and gases) experiences a retarding force fluid friction or viscous drag. If the moving bodies are spheres then fluid friction F is given by
F = 6 π η r v
Where η is the coefficient of viscosity,
Where r is the radius of the sphere,
Where v is velocity pf the sphere.
When the fluid friction is equal to the downward force acting on the sphere, the sphere attains a uniform velocity. This velocity is called Terminal velocity.