Friction is a force between two solid surfaces which are in contact with each other and which opposes the relative motion between the two. The friction arises because of two reasons. One is, that the surfaces are not smooth on a microscopic level and that leads to rubbing of the two, producing a force opposing to the motion. Second is the interaction between molecules of the two surfaces.

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If the interaction is attractive, the motion between the two surfaces is opposed. The frictional force can never be eliminated but can be reduced by making the surfaces smoother or by inserting some smooth material (like power or liquid) between two surfaces. The force is considered to be independent of the relative velocity between two surfaces but this is an approximation.

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What is described above is sliding friction.There is another concept of rolling friction where when two bodies are moving relative to each other, the surface in contact is stationary. This happens when a wheel is rolling on a plane. Here the part of the wheel which is in contact with the plane is not moving and therefore there is no sliding of the two surfaces. But there is still some frictional force because when the wheel rolls, the part of wheel which is in contact with the plane has to be separated and that requires some force and this force opposes the motion. Rolling friction is much smaller than sliding friction. Finally, when a solid is moving in fluid (liquid or gas), there is a force opposing the motion of the solid. This force is not called friction but viscosity.

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Types Of Friction Force-