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Hi Aydin Angle of internal friction can also be thought of as the angle of Repose. ie, take a dry granular material like sand, and pour it onto a flat surface. The cone of sand will form up at an angle of repose determined by the internal friction of the material ie the friction between the particles. By External Friction, I infer you mean the friction between say that same sand and another material, such as the rear face of a concrete retaining wall. This will obviously be greater than the friction between that sand and something smoother like say glass. ============== Best regards, Simon Woodward Chartered Professional Engineer