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- Dry friction: Dry friction is also known as Coulomb friction. Dry friction occurs when the non-lubricated surfaces of two solids are in contact under a condition of sliding or a tendency to slide. A friction force tangent to the surfaces of contact occurs both during the interval leading up to impending slippage and while slippage takes place. The direction of dry friction force always opposes the motion.
- Fluid friction: Fluid friction occurs when adjacent layers in a fluid (liquid or gas) are moving at different velocities. This motion causes frictional forces between fluid elements and these forces depend on the relative velocity between the adjacent layers in a fluid. When there is no relative velocity, there is no fluid friction between adjacent layers. Fluid friction depends on the velocity gradients within the fluid and the viscosity of a fluid, which is a measure of its resistance to shearing action between fluid layers.
- Internal friction: Internal friction occurs in all solid materials which are subjected to cyclic loading. For highly elastic materials the recovery from deformation occurs with very little loss of energy due to internal friction. For solid materials which have low limits of elasticity and which undergo appreciable plastic deformation during loading, a considerable amount of internal friction may go with this deformation. The mechanism of internal friction is associated with the action of shear deformation.