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Why is there more friction on ultrasmooth surfaces?

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This is true to a point (pun intended). For rough surfaces, the contact area is much smaller than the entire surface area. read more

If the two surfaces of the same material are very smooth, the molecules and atoms of one surface get closer to the molecules and atoms of the other surface. As they get closer the molecular cohesive forces begin to take effect and the two surfaces will actually stick to each other. read more

Friction refers to the forces that resist the relative motion of two surfaces sliding against each other. There are three major causes of friction - molecular adhesion, ploughing effect, and surface roughness. Molecular adhesion refers to the forces that attract molecules to each other. This is the most important cause of friction. read more

The normal force is always directed perpendicular to the surface, and the friction force is always directed parallel to the surface. F F and F N are always perpendicular to each other. The force due to friction is generally independent of the contact area between the two surfaces. read more

Friction between rough surfaces occurs because, on the microscopic level, the surfaces have high and low area like hills and valleys. When they move horizontally past each other the hills of one surface interlock with the valleys of the other, similar to the way in which the teeth on gears interlock. read more

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