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Interlocking of microscopic surface irregularities (asperities) — primary cause
Molecular adhesive forces (van der Waals forces between contact surfaces)
Normal force — greater pressing force = greater interlocking = more friction (f = μN)
Nature of materials — rubber/concrete = high friction; ice/ice = very low
Surface deformation — soft materials deform to increase contact area
Lubricants — reduce friction by creating a separating film

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